You select a Project / Branch like normal and then you can select any level in the directory structure for that Project / Branch.
When you click button "View Branch", you will see the files from Subversion under that directory that were changed since the branch was created.
If in the column "Build or Requested" doesn't have a value than it was never in a request or in a build. If it has a value that starts with an "R", like "R - 4/17/10 19:17", then it was in a request, but never built. If it has a value that start with a number, like 114, then this is the build that that file was in.
At the bottom of the page will be a listing of the files that were changed since the branch was created, but not in a build. This is important to know when you go to collapse the branch.