pcad,

PowerAgent on the Mac doesn't deal with the Garmin 500, at least (I'm going to assume the 800 is the same way, since it's new and Garmin's choice these days is the .fit file format rather than the previous .tcx file format).

The version of PowerAgent I have, 7, doesn't import .fit files directly. If you convert the .fit files to .tcx files, either with Garmin Training Center or downloading from Garmin Connect then you can import fine into PowerAgent. I just did that with today's ride and it shows up right in PowerAgent, including the correct power data, etc.

PowerAgent is a pretty crappy piece of software, though. There are better choices out there.