Try using one of the views that the TFS database has. I've read in one site that you can use WorkItemLatestAndWere (TFS2010) and I found it useful for most of my uses. I am not so sure about creating and/or accessing a cube - it sounds like more work to me if you can access the database tables anyway and you can use Qlikview to design and construct some sort of data store architecture by creating and storing qvds. Just my two cents.
A good article from Microsoft regarding the 3 layers of data TFS has is here. I would bet that accessing the "Relational Warehouse" layer would be the best bet for tapping in to TFS data. It avoids the complexity of the OLTP layer and the difficulty in passing MDX language to the pre-built cubes.