To activate cheats while playing Dawn of War, you first need to have the version 1.10 patch. Once you've made sure you have that, continue.

During a game, press Ctrl+Shift+~ to bring down the console. Type one of the following:
cheat_requisition(####) - gives requisition resource equal to the number that replaces #### (you must include the brackets)

cheat_power(####) - as above, but gives power.

cheat_revealall - reveals the entire map.

cheat_killself - kills yourself.

Once you've entered the cheat, press Ctrl+Shift+~ again to close the console and return to the game.

Note: You must turn the cheat option on in Skirmish mode. Cheats also work in multiplayer (I believe), but only so long as the game creator enables them. You won't get away with cheating online without anyone knowing: When you enter a cheat code, a skull and crossbones icon shows up in the message area, with the word "Cheats!" next to it.