Can I use PM 5 with Mac 10.5.8?
No. Photo Mechanic: Version 5 runs on Mac OS X 10.6.8, 10.7, and 10.8.
Mac OS X 10.4.11 and 10.5.x are not supported. The most recent version of Photo Mechanic that supports those older OSes is Photo Mechanic 4.6.9.
-Kirk