Drill up and down refers simply to clicking the plus signs on dimension items that open up to more detail. For example, by clicking on the plus sign by 2011, you open it up to the 4 Qtrs. If you click on the plus sign by one of the quarters, you open it up to the months.
This differs from drill through, which allows a user to see the rows that make up a data point. If you google "SSAS drill through", you will get instructions on what to do.