Plaxo for Mac does not directly synchronize with Entourage. However, through OS X Sync Services (version 10.4 and higher only) you can configure Entourage to synchronize with your OS X Address Book, which then synchronizes with Plaxo. Here’s how to set it up:

1. Make sure you have the most recent version of Entourage 2004, version 11.2.3. To check which version you have, start up Entourage, and go to “About Entourage” under the “Entourage” application menu. If you have an older version, an upgrade is available free of charge from Microsoft.
2. Once you have a current copy of Entourage setup, go to “Preferences…” under the “Entourage” application menu.
3. In the Preferences window, under “General Preferences”, go to the “Sync Services” preferences.
4. Check the checkbox labeled “Synchronize contacts with Address Book and .Mac”
5. Select the Entourage Address Book folder you wish to synchronize with.
6. Click “OK” to exit the Preferences window.

Entourage will now automatically sync through OS X Sync Services, which will connect to Plaxo through your OS X Address Book.