Ok. For timebomb, you can use search in here for timebomb script. Should be few of them.
There can be better ways, but here is one way. First you place trigger, size and activated for what you like. You can put timeout, example 300 to all min/max/mid so its 300 seconds (5minutes) timeout. In activation line, you can type: hint "5 minutes time left...
or what you like. You can make also other triggers, where is warning for "4 minutes left..." or "1 minute left...". Most important is, that last trigger what ends mission, is marked for Type: End-1. It can be after bomb exploded or something if you wish.
"hint" is that small screen in up-left corner, witch pop-out whit "ping" sound. You can find also scripts where player can check how much you have time left, or trigger/scripts where hint appears every minute (or something), how much you have time left. You can use also that white text, as called "titletext" what appears in center of screen: titletext "you have 5 minutes left...
PS. I prefer to use hint rather than titletext. It looks much better.