This would be and OS thing. You did not say how much ram you have installed, perhaps it is 3 gig?
Check you computer readout to make sure OS is seeing all the ram you have.
Hi Curt -- Thanks for the post and for point-out my ommission. I have 8GB of RAM installed, and the About This Mac menu confirms this is the case.
Hello, you need to check if Photoshop was not set to start in its 32 bits mode: http://helpx.adobe.com/photoshop/kb/plug-ins-photoshop-cs5.html#main__How_to_run_Photoshop _CS5_in_32_bit_and_64_bit_modes
Having the same problem.
CS v5 - MacBook Pro - dual 2.3ghz Core i5 - 4GB RAM
Running PS in 64-bit mode
PS shows only 1.488GB of available RAM to use.
Verify it's x64 by doing Help - About Photoshop and looking for x64 in the About dialog. If it doesn't say x64 there, you're running the 32 bit flavor.
The setting for changing the max RAM you're allowing Photoshop to use is in the Photoshop - Preferences - Performance dialog. Note that 64 bit or no, it won't let you use it all up, as it knows your system needs some of it to run. But you can probably give it more than you are at its current settings.