VersionTracker links to an ancient file, which you should not bother with. I finally found the direct download link for the current version of the Plaxo toolbar for Macs (v. 3.7.8) --

http://down.plaxo.com/down2/mac/late...ac.zip?lang=en

So, you should not have to waste time logging in, etc. just to grab this latest post.

From the ReadMe file:

System requirements
Plaxo for Mac requires Mac OS X version 10.3.9 or later and an active internet connection to sync your data. iCal sync and Apple Mail plug-in require Mac OS X version 10.4 or later.

Installing Plaxo for Mac
1. We don’t anticipate any problems, but please backup your Apple Address Book and iCal calendars before you continue with this installation as a safety measure.
2. Double-click the Plaxo for Mac installer and follow the instructions that appear.
3. Don't forget to check out Plaxo.com, where you can always access your data.

Known Issues:
Smart Groups are not usable as Groups to Omit from Sync.

Feedback:
Email us at support@plaxo.com

Plaxo Forums:
Check out the Plaxo for Mac forum at http://forum.plaxo.com

Revision Notes:
The revision notes for Plaxo for Mac can be found at http://down.plaxo.com/down2/mac/releasenotes.html