So you're trying to add a fact table to a cube that you've already created, but you can't figure out how to do so without recreating the cube from scratch? Here's what you do: In the Measures pane in the Cube Structure tab, right-click and select New Measure Group. Then select the fact table, and voila! (This is assuming you've already put the fact table in the data source view.)
addendum: Looks like the standard way to add a dimension to the cube is to right-click on the cube name in the Dimensions window in the Cube Structure tab and select Add Cube Dimension.