I understand that you may not be too good with computers but this couldn't be simpler.
If you download a mission then unzip the file you'll get a .pbo file. Then you just have to put it with the other mission .pbo files in the Flashpoint folder.
C:ProgramFiles/Codemasters/OperationFlashpoint/Missions
Something like that, I'm on the Lap Top so I can't check. With addons it just the same but the addon goes into the addon folder. Guys who might have a handgun go in the res/addons folder, or they do on my computer.
When you want to play with an addon you just go into the editor on the main menu and then choose a map, click units and choose the unit you want to play with.
You're lucky that I got in and answered this because it isn't really something we help with here. BIS forums is the place for that and this topic will probably get locked.
Here's a link for them,
http://www.flashpoint1985.com/cgi-bin/ikonboard311/ikonboard.cgiAfter you've played with the addons you've downloaded, try making a mission. Once you get into the editing side of Flashpoint you'll see why we still play it so long after it came out. Its in the editing side of things that the game really shines.
Here's the best site for addons in my opinion.
Ofp.info
Enjoy.