1. Click the re-author button.
2. Browse to the files you want to re-author.
3. Drag and drop from the right pane to the left (or maybe double-click to add I think).
4. Right click the files in the left hand side and choose to select start and end frames.
Viola!
I'm writing this from memory coz I'm on the work PC so it may not be exact, but that's the gist.