Try this: (if you haven't already tried it)
Backup every theme installed.
Nuke everything related to WB, via Impulse.
Remove the WB folder manually (if not done by Impulse)
Reboot.
Start Impulse. Reset data (Impulse orb -> prefs -> reset data -> 'no' to restart Impulse)
Reboot.
Start Impulse, and, hopefully, you should see WB 7
(Remember to have 'Show Pre-release versions' checked. impulse orb -> Show Pre-release versions)
If that doesn't help. Try nuking Impulse, reboot, and reinstall Impulse.