1. Edit your hosts file with this entries

127.0.0.1 pd6.blist.parallels.com
127.0.0.1 pd7.blist.parallels.com
127.0.0.1 pdfm7.blist.parallels.com
127.0.0.1 registration.parallels.com
127.0.0.1 parallels.com
127.0.0.1 update.parallels.com

or a better solution: use little snitch!

2. Enter this serial:
KM7SWA-D0V7WH-40CFX7-WS815F-YV0QRR

3. Enjoy ;)