If you are the creator of custom POSTAL 2 content from before the Steam updates, and want to know how to get your stuff working again, look no further! I'll go over some of the radical changes that have taken place in the codebase.
Pretty much anything that's not a code package should work out of the box. Copy over the necessary files from your old copy of POSTAL 2 and into their proper folders in POSTAL2Editor.
Try running your map. If the map's prefix is "cus-" or "ws-", you can launch POSTAL 2 from the command line or Explorer, go to the Workshop Browser, and try running the map from there. If not, you'll have to launch it by name from the command line.
If your map loads, playtest it and ensure everything works as before. If it does not, pay attention to the error message, if there is any. It will give you valuable information on where to turn to. You may need to change a static mesh or texture reference somewhere.
If you try to load a map and instead end up falling into the "startup" fire box, don't panic! Press the tilde (~) key to bring down the console, and type in "quit". Then, load postal2.log. Your map failed to load due to a missing package, and postal2.log will tell you exactly what package is missing. Copy it over and try again.
If your map contains custom "baked-in" (MyLevel-based) UnrealSript and crashes, see the next section.
If your mod contained .U files, read on -- your task is probably going to be a bit more difficult.
First, go ahead and give it a shot in the dark -- simply copy over the .U files and try to use the weapons, gametypes, or other content within. You may get lucky and it might work with no modification. If it does not work, read on.
To make your old .U files compatible, you'll need to dig up the UnrealScript source of your mod. Assuming you are indeed the mod author and you have those scripts, copy them into POSTAL2Editor, set up Postal2.ini accordingly, and try to recompile your mod.
If you've lost the scripts, you can get them back by exporting them from POSTed. However, there are a few caveats with this method:
You must export the scripts from the 1409 version of POSTed. The 1415 version of POSTed will not load them (if it could, then you wouldn't need to go through this baloney)
The default properties section will be mangled
Object-based properties, such as emitters, will be lost. There is a bug in the Unreal editor causing these objects to not be exported along with the default properties. You will need to reconstruct them from scratch or copy them from the actor properties window.
Once you have your scripts, try recompiling them. In most cases they should compile without a hitch. AW-based mods may run into some trouble, though: a lot of AW actors changed packages. If you have any "hard" references in your actor properties (where the package of a class is explicitly states), you may need to go back and change them to "soft" references (where the package is not explicitly stated). Look at ucc.log after compiling to see if you've run into this situation.
If you still run into errors, check the 1415 UnrealScript source to see what changed. Make the necessary changes to your script and recompile.
If you cannot resolve the issue on your own, ask for help! There's bound to be someone listening on the Workshop forum.