drush_mm can't find drush

$ drush enable drush_mm
Module drush_mm cannot be enabled because it depends on the following modules which could not be found: drush    [error]
There were no modules that could be enabled.                                                                     [ok]
An error occurred at function : drush_pm_enable                                                                  [error]

I have drush sitting at /usr/bin/drush, and I tried putting a symbolic link to it in my modules directory.

admin->build->modules shows Depends on: Drush (missing).

Not sure if this is drush_mm's fault, or if I just don't have drush configured correctly. For the record, it works great in my 6.x site on the same server.