you can use Soundflower (free) to route any output to the ProLevel input.
http://www.cycling74.com/products/soundflower
Download page:
http://www.cycling74.com/downloads/soundflower
also check out the ProLevel (ProPhase) forum page:
http://forum.katsurashareware.com/
-author
ProLevel
real-time stereo level meter with clip indicator
Version: 1.2