Which version are you using? Since CP7 there are two new interactions: Hourglass and Timing that allow you to trigger a jump action on elapsed time directly from the interaction. No use for an advanced action. I don't see the link with an action on a master slide.
If you want to stay with the old Timer widget (or if you use a version before 7), have a look at one of my old blog posts: Timer widget... to stress your learners - Captivate blog