Running out of CPU Power error while using Pro Tools 9

Hi,
I was a newbie Pro Tools 9 user on a Mac. Actually, it was my first time to tinker on an audio software.
Anyway, I got this error message (see below) while using it, and I didn’t know what to do.
Can someone from the forum give a step by step guide on how to resolve the issue?
Hope he is patient enough to help me. Thanks.

You are running out of CPU power. Remove some RTAS plug-ins or increase the CPU Usage Limit in the Playback Engine dialog.
(-9128)
