Make sure no other FireWire devices are attached before you begin troubleshooting.
All of the PreSonus FireWire based mixers and audio interfaces should show up on the FireWire device listing in System Profile.
- Applications > Utilities > System Profiler (or System Information, on newer OS X versions)
- Look under Hardware > FireWire to see FireWire devices
If you don't see the mixer listed here then you'll need to do some troubleshooting. Although there is a great guide over on the Knowledge Base portion of the site, I'll summarize here.
1) If the mixer is showing up in the System Profile then you probably have a driver issue.
(Find the right driver for your system here. Make sure you read the release notes.)
2) The mixer doesn't show up on the FireWire device list or if you see an error in red then it's probably best to contact support.