If you wish to use BirdieSync 2.0 - 2.3, it is necessary to upgrade your license. Registered users of BirdieSync 1.x may upgrade to this new major version for half the original price (9.95 €). Only users who have registered their version of BirdieSync in the last 3 months before the official release of BirdieSync 2.0.0.0 (so from the 26th of October 2009) are offered a free upgrade.
You can upgrade on BirdieSync web site with your previous registration key:
http://www.birdiesync.com/upgrade

Note that it's not possible to enter your license key if you still have a beta version of BirdieSync 2.0.