Me too. This with the 4/3/08 creation date version of V5.3.2 running under OSX 10.5.2.

In Now Contact, clicked on File/Synchronize/With Address Book and immediately got a dialog box with an icon of an Exclamation Point on a Red Stop Sign saying "Address Book Sync could not run - Address Book Sync is not supported on Mac OS 10.5 or newer."

Tried a different contact file, but got the same result.

Trashed the com.nowsoftware.nowcontact.plist and com.poweronsoftware.nowcontact.plist files and got the same dialog box.

What can we try next?