After reading some of the threads about LP MB and LP EQ, I decided to try them on a couple of my projects. They are mostly just recorded audio with minimal processing. When I inserted the LP MB into one of the projects, it responded beautifully - I tried one of the mastering presets and it sounded VERY good to my ears. When I inserted it into another project with only three audio tracks and minimal processing, it would not respond at all to audio. I can select any preset I like, but it will not DO anything to the signal. 
 
I suspect it is a "newb" problem - some sort of project configuration or something - but have tried a few things and gotten nowhere. Any thoughts on what might make the LP MB not respond to audio when placed on the master bus (or anywhere if it's not really related to the bus)?

Could it be that the fx rack is bypassed?
Toggle the "button" next to the bus name.

There is also a global FX button in the Mix module in the Control Bar.

Additionally, if the level of audio hitting the bus is too low to trigger any of the compressor thresholds, you won't hear it do anything.

Jeff M. and John: I will check those things out when I get home from work and report back. Thanks!
Sanderxpander: If I can hear the audio coming out at a "normal" level, would that be enough, or at least prove that there is enough to trigger the thresholds? One of the reasons I decided to try it is because I am trying to expand my usage of the tools I have available, but there is SO much that I don't know yet... 

Well you can lower the thresholds to try it out. To be honest it's unlikely that this is the problem but it could be one reason why it's working on one project and not on another.

Well, as I expected, it was a newb problem. In the console view, in the FX bin of the Master Bus, I had inadvertently clicked the button that looks like a power button, thus disabling the FX bin. Thanks for your help, everyone. :)