Seriously. STOP SPAMMING. Maybe you could have just put all your questions to one thread, instead of four. You asked this once, and it's going to get locked.
But let me try and explain one more time!
1. Find a mod from the Mods forum.
2. Click the download button.
3. Open the downloaded file, which is usually (modname).rar and EXTRACT the files to Cortex Command folder, with a program like WinRAR, Power Archiver, Winzip.
4. The mod will go to cortex command folder. There are 3 folders already in that place. They are called: Base.rte, PromGrounds.rte and Test.rte. Your mod should create a fourth folder, called (whatever mod name).rte
5. Now click the cortex command.exe ( shortcut ). The thing you click when you want to play. Start a new game, and you should see your downloaded mod in the buy list. You have to order it, before you can use it.
Get it? If you don't get that, then pshhhhh.