You can download the 4.13.2 (current Beta version) using your Pro personal download page It should say "New Beta available"...click on that.
Or, if you don't have your download page bookmarked, go here & enter the email address you used to buy Pro. Providing you bought Pro less than 6 months ago, you'll be able to download the Beta Pro version.
http://sales.limewire.com/support/?

If your Pro's out of date, you can download the basic Beta version from the basic download page
http://www.limewire.com/english/content/download.shtml