Blender Problems with rotating grid with middle mouse button,HELP?

Hello, I,m new to Blender and have an issue with Blender Rotation with middle mouse button in 2.5,does not work! I’ve tried going to properties and making many changes,but still does not work!In 1.5 (MMB) works fine,but like the set up in 2.5 better. Is there any ideas on how to fix this problem?
I’m running Powermac Dual 2 GHz PowerPC G5 with Leopard Version 10.5.8.
Thanks for any help in this matter, art101

The setting for mouse rotation is found under User Preferences, Input / Orbit Style. Which Orbit Style do you have selected?