When you purchased the PRO version, you should have been given a personal web link (installer) to use for your 6 months of free upgrades. If you didn't save this web link.... you can click on "Lose your installer? Get Support" link on the PRO download page. You can then enter your email address originally used when you first purchased PRO. LimeWire will re-send your personal link for the free PRO ugrades.
Save this link.... for the remainder of your 6 month period!