There are two ways to make compressed iron. Blowing up a dropped stack (of any size) of iron ingots on the ground, or using the pressure chamber. The pressure chamber, however, requires compressed iron to craft, which means that you must use TNT to get started on PneumaticCraft. If your server's administrators have installed a anti-griefing plugin that makes TNT-compressed iron not work, you will have to work something out with the administrators to make it happen.
You may have to point out that without some form of fix for this problem, they might as well disable PneumaticCraft, because you cannot progress in the mod at all without compressed iron.
The official PneumaticCraft wiki has some information on mod progression, and is the source of my assertion that there are only the two ways to make compressed iron.