I downloaded the mainframe 2.0.pk3 file into gamedata\Mainframe 2.0 adn then launched the game. I went to mods and there was nothing there, I also tried it with just having the file in my gamedata folder. Any help would be appreciated thanks.
CRAP! wrong forum.
have you tried the base folder?
All .pk3 mod files should go into the GameData/Base folder. The installation instructions should be inside the readme of the mod you downloaded.
IIRC, you can put it in a different folder in /Gamedata. We'll call it "Mod". Then, if you go to options in SP, you can load it as "Mod." AFAIK. Otherwise, /base will do fine.