If you're running Windows XP and you've recently got the Steam version and want to check out the Workshop games, or create some yourself, you will need to make sure that your system has an entry for LOCALAPPDATA in its System Variables section. If you're on Windows Vista/7/8 this is not required as Windows sets it for you.
An easy way to check if you're ready is to click your Start button, choose Run, type "command" into the run prompt, and then type "set LOCALAPPDATA" (without the quotes) and press Enter. If your variable is set already it will return a path to a folder; if not, you will get that it is undefined.
Right-click My Computer, and then click Properties. Click the Advanced tab. Click Environment variables. Click New to add a new User variable. Give it the name LOCALAPPDATA Give it the value %USERPROFILE%\Local Settings\Application Data Now just okay your way back to the control panel and close it.
If you had GameMaker or the Workshop player open you'll need to close and restart them, otherwise you should be good to go.
We will be updating the Steam version in the future to support this variable not being set.