I am not entirely sure about that error but I got the error when I accidentally clicked it twice, so you may wish to check your database to see if the tables have already been inserted.
deleted & re-loaded site, still same error when trying to load sample data.
Only clicked on it once.
Was able to continue with setup then access site but with no data included.
Strange....
Found something interesting.
Thought I would check out the version of Joomla in the Rocketlauncher downloads.
The ones giving the XML error seem to be 1.5.2, but those with 1.5.3 seem to work just fine and install the sample data.
Looks like a bug in 1.5.2. Might be something to look at in packing the downloads with 1.5.3 if that's something that could be done relatively easily.