If you're like me and want to import your current save file to the mod, google wasn't much help so here's a quick how-to:
> Once you click "Subscribe" Steam will download the mod and create the folders for it.
> Once it's done, run TL2 and select to play with mod's, Synergies should be listed on the left side of the window. Make sure the box next to Synergies is checked and start the game.
> Once in game at the main menu, click on new game and just choose whatever, or make a Necro or Pally if you plan on playing one. Creating this character will create a modsave folder.
> After you've made your toon you can save(if you want) and exit the game and copy/paste your save file(s) from save to modsave and you're done!

My TL2 files/folders were located at (yours should be too):
C:/User/YOURACCOUNTNAME/My Documents/my games/runic games/torchlight 2
If you're files aren't there, then when you load up TL2 to select the mods your playing with, there should be a button near the top that lets your browse your mod folder; that will show you where your files are.

You're pre-mod save files are in a folder called save.
Within that folder there should be another folder that's just all numbers and jargen, you're save file is in there. You're looking for a .SVB file extension.

Also in this folder is things like your shared stash (sharedstash_v2.bin), key-bindings (keybindings.dat) and your general settings (settings.txt). You can copy those over if you want to as well.