I can,t figger out how to detenate the explosives in some of the maps?

You have to have tnt or a satchel charge in you're posession and go to the spot, where it says "Place charge" on the left side of your HUD. Simply walking to the spot will place the charge.

One thing to note is that some walls can only be blown open by one team and some spots require you to stay there a for couple of seconds to place the charge.

And of course, some things can only be blown up by bazookas. It basically depends on how the guy who made the map programmed it.

I recommend you try these things out by starting your own server. dod_caen for example has lots of things to blow up.

http://www.mardymouse.co.uk/modules.php?name=Content&pa=showpage&pid=10

This site has a detailed set up of blowing walls up.