Perhaps this will explain?
- Logic Pro 9.1 and MainStage 2.1 require Mac OS X v10.6.2 or later and an Intel Core 2 Duo processor to run in 64-bit mode.
Later versions of Logic/MS need a later version of the OS, but if you don't have an Intel Core 2 Duo you are out of luck.
Note the difference...
There is a mobile "Core Duo" processor that is 32-bit only
It's the "Core 2 Duo" that is 64-bit capable.