It is unfair to blame a company for stupid users.
It's also unfair to blame users for a stupid company, AppData is a hidden directory.
I have looked everywhere pretty much and I even looked to where the mod path is and even when I did I still couldn't find it.
I'm not running the newest version of Sins, but unless something changed drastically the mod folder should be in
Win 7: C:\Users\Username\AppData\Local\IroncladGames
WinXP: C:\Documents and Settings\Username\AppData\Local\IroncladGames [iirc, I haven't used XP in a while]
As for the game files themselves, installed through steam should be in:
%steamroot%\steamapps\common\ where %steamroot% is wherever you installed steam to, usually defaulted to C:\ProgramFiles\Steam on 32bit windows or C:\ProgramFiles(x86)\Steam on actual windows.
As for hidden folders, you can expose them on Win7 by selecting Organize -> Folder and Search Options -> View Tab and then selecting "Show hidden files, folders and drives."
It's a similar story for XP, but the option is in the edit drop down menu if I recall.
Hope this helps.