I'm on Mac. What do I do?
While these methods require more effort than the Windows ones, they've been confirmed to work.
Update Jan 9 2014: It seems the save files are now at least partially synced across platforms (although I'm not quite sure how that's possible), so just running the game on Windows might do the trick. It worked for me at some point, but then I had to go back to swapping save files. When I finally beat the whole game I got some of the achievements I had unlocked by just running the game on the Windows partition of my Mac, but even then I had to replace the files to unlock one final one. So even if just running the game on Windows works for you, you might still need to use methods in this guide at some point.
Obviously you will need to have access to a computer running Windows. This guide however won't cover how to setup a Windows partition on your Mac, and installing Windows on your Mac for the sole purpose of unlocking couple of Achievements in a single game would be questionable anyway, so let's not get into that. Let's just assume you already have Windows installed, or a friend of yours is letting you use their computer. I'm sure you can work this out somehow.
It worked once for me, but then I had to go back to switching save file. Don't know what's up with that. You'll need to run the game once anyway on Windows to create the save files anyway, so you'll end up testing this method anyway. If it doesn't work, see methods 2 and 3.
This method will require you to replace two files, serial.txt and so.sol, on Windows with the ones that store your progress on Mac.
On Mac the files can be found at the following locations:
~user/Library/Application Support/Steam/SteamApps/common/the binding of isaac
C:\Program Files\Steam\steamapps\common\the binding of isaac
Application Data\Macromedia\Flash Player\#SharedObjects\<Numbers&Letters>\localhost
The location of Application Data folder may vary depending on what version of Windows you're using. Your own user folder on C: drive is a good place to start looking. If you can't seem to find it, try enabling "show hidden files and folders" under Folder Options. Then, using Run Command (Windows key + R, or if you're running Windows through Bootcamp on a Mac, cmd + R), go to "%appdata%\Macromedia\Flash Player\#SharedObjects\". This works on Windows 7, don't know about other version.
Find the files on your Mac, and store them somewhere, for example in an USB memory, or upload them online, or use any other means you find the most convenient for yourself as long as you will have access to those files when you're on Windows. Dropbox has been proven to work quite well for this purpose.
Since serial.txt is located in The Binding of Isaac's Steam folder, you obviously need to run the game at least once on Windows for you to be able to replace the file. Once you're on Windows, install The Binding of Isaac from Steam and run it at least once. Then locate serial.txt and so.sol, and replace them with the files you copied from your Mac. Needless to say that you can make backup copies first if it makes you feel more comfortable. Once you're done, run TBoI again and you should start seeing some Achievements popping up. Yay, Achievements!
The downside of this method is that you will have to do it over and over again every time you want to unlock the achievements you've earned, so it's not advisable to start juggling with files every time you unlock something. Also, since serial.txt and so.sol only store your progress on the game, all your achievements will appear to be unlocked at the same time on Steam.
An easy and fast way to unlock your achievements is to install and launch the game through a Wineskin wrapper meant to run Windows-only Steam games on Mac. I don't see why any Steam wrapper wouldn't work but in this example I will be using Dota 2 wrapper from portingteam.com
because I have personally confirmed it to work. All credit goes to user Lune
for pointing out this method.
I really don't have any knowledge about Wineskin wrappers so there are a couple of occasions where I suggest doing something that might not be necessary, but this is how I managed to unlock one achievement I didn't bother rebooting to Windows for. The Dota 2 wrapper is pretty large in size, almost 577,7Mb zipped and 1,43Gb unzipped, so you might want to try some other wrapper if you have slow internet connection. Just make sure it's designed to run Steam games.
- Download any Steam wrapper[portingteam.com]. The download page doesn't have any useful information unless you plan to use the wrapper also to play Dota 2, so you can just download the file Dota 2.app.zip, close the page and never think about it again. I'm not sure if its necessary but I unzipped the wrapper in the ~user/Applications because that's where my TBoI launch icon is.
- Opening the app will ask you to log in to Steam. I would recommend closing your Steam for Mac app before logging in through the wrapper. Just to prevent any possible problems. Again I'm not sure if it's absolutely necessary but hey, better safe than sorry, right? It might take some time for the Steam window to pop up, and it might try to download some updates but I just canceled those.
- Once you're logged in, go to your Steam Library and install The Binding of Isaac. In my case it took some time to prepare the files for install, around five minutes I think, so just be patient.
- Launch the game and you're done. I didn't see any achievement pop ups but my Steam did update my achievements right after shutting the game.