Hey, welcome to the forums. You need a certain level of score for each kind of attack first of all. I believe the numbers are: 90-mortars 100-artillery 110-airstrike 120-napalm After you reach one of those, you have to wait 45 seconds before you can launch. A message will come up telling you when you're ready. Press "B" to bring up your binoculars, Aim, and press "F" to fire.
Other Mods Other mods also use airstrikes and such. I just want to note that they use other methods such as clicking fire to call the air strike. Almost all the mods have information on how to call airstrikes in. Im sure that they will all involve using the binoculars.