Do you want to use custom tones that you've created and copied to the phone? Or, are you using a custom tone app that you've installed? I'm using custom tones that I created and copied to the phone as Notification tones (in MP3 format). If you're using an app, there might be a problem if the app doesn't know where to store them (internal storage verses external card).
If you use one of the file manager apps (I use Androzip Pro), the on-board storage shows up as a folder named "./sdcard". The actual removable SD Card storage shows up as a sub-folder to that named "./sdcard/_ExternalSD". I created a folder in the internal storage named "./sdcard/Media/Audio" and added three sub-folders to that named "Alarms", "Notifications" and "Ringtones". I copied my custom tones from my PC to these folders and they show up in the pull-down list and can be assigned.
Alternatively, when you connect your Revo to your PC, you'll get separate drives (letters) for "INTERNAL" and "SD CARD" storage. You can create the folders from your PC on the "INTERNAL" storage at the root as "Media\Audio\Alarms", "Media\Audio\Ringtones" and "Media\Audio\Notifications" and copy the custom tones to the respective folder. - MarkC