Now look I am a long time FPS person and I have GRAW 1 and now ver 2 so I know how to place a charge but on this level it only shows me that I can place it on the chopper but when I hit the key to do so...NOTHING!!! I have killed all AIs and I have the other guy with me so what am I missing?

You have to have a demolitions on your team and put in the bomb to do so. Make sure it's the bomb that doesn't blow on your command, it has to be from the mission, after your set the charges for the chopper, run the hell out of there. If you think you can't outrun them, put a claymore mine or something like that on the barricade, enemy forces will go through there and you will buy yourself some time, the tank however is ignorable since it will take some time for it to get to you.