If this is the first time you're going through the Weekly Episode with this character, you can always open up your Missions List (hit 'J'), and click on the Available tab. This will list out the missions you are eligible for. Click on Colosseum and Bob is, in fact, your Uncle.
If this is the second or more run through Colosseum on a given character, visit your bridge and go to your Ready Room. If this is a Federation character, the mission will be found under Replay, Romulan Front, Featured Episodes, Colosseum. If this is a Klingon, you just have to go to Romulan Front.