The Sansa port of Rockbox doesn't have full USB support yet.
If you want to copy anything to your player, you have to boot into the original firmware and do it from there; just make sure your Sansa is configured to use MSC (USB Mass Storage) mode.
There's no definite timeline as to when USB support will be implimented in the Sansa port of Rockbox, so please be patient. Just use the above method for now.