I ran into the same problem as you. My Clip+ rockboxed would not be recognized as usb mass storage.
I downloaded the current build for the Clip+. I powered off my Clip+. Then I pressed the power button while holding the Left key and booted into Sansa firmware. I connected to my PC. Then I extracted the .rockbox folder from the downloaded current build to my Clip+ overwriting the already existing .rockbox.
Then I disconnected and powered off the Clip+. When powered on into Rockbox, I connected it to my PC and saw it recognized as a usb mass storage. A driver was automatically installed and I could transfer music to the Clip+.