This is a common problem with Studio Compiler and also Source SDK. I'm not sure what causes the problem but one way of fixing it is by editing your GameInfo.txt files. In your cstrike directory you will find a file GameInfo.txt -- open it up. Near the top you will see:

ToolsAppId 211

All you need to do here is put // in front of it to comment it out. Save & Close, then try starting Studio Compiler. If that didn't solve it you might try the other GameInfo.txt for Source SDK. Go to username/sourcesdk/launcher/ and try the same steps with the GameInfo.txt there.

-- Can't load FileSystem_Steam.dll even when present in folder

This error doesn't make sense, but it only happens to me when I try to load Studio Compiler from the sourcesdk/bin folder. If you're using this tool for Counter-Strike: Source then StudioCompiler.exe needs to be in sourcesdk/bin/ep1/bin instead.

-- Can't find steam app user info.

This is another common problem. What you do here is you go to your Program Files/Steam/config folder and copy the SteamAppData.vdf file you see there. Now go to your sourcesdk/bin/ep1/bin folder and create a new folder named config if it does not exist already. Paste SteamAppData.vdf into there and try to run Studio Compiler.

-- vstdlib.dll not found.

Basically what you get here is that it can't decompile the model and there is a .smd referenced with a weird name -- usually a strange character like a box or triangle at the end. This could mean one of a few things, but the main one is that the model was compiled for another hl2 mod. A possible fix that won't work every time but might work is to try opening the model's .mdl file with a hex editor. You can get one for free from google.

If you open the model and the first bit of text reads:

IDST...

This might be a solvable problem for you. Re-write it like this:

IDST,...

Studio Compiler might be able to decompile the model now. This might not work every time, but it's worth a try.