Re: Bombs in DS
First, you need the bomb entity. can't remember off-hand what it's called. Second, the player has to have the defuse kit in his inventory. Next you need a brush set up as a trigger_useTool (i think that's what it was called) surrounding your bomb and large enough for the player. Set up the usetool trigger to require the defuse kit, set the timer, etc and rig it up to remove the bomb when the time is reached on the trigger.
Sorry I can't be more specific than that. I don't have the tools sitting around to dig through.