6.5 Has the search engine friendly URLs built in.

Before Infopop was "inspired" by it and added it into the latest version, I was the one to create, test and perfect the mod. I sold it through ThreadsAddons.com
http://www.threadsaddons.com/index.php/paypal/faq#2

There are lots of new features in 6.5 - so if you upgrade, you'll get lots of new toys to play with, including the SEO urls.

I don't have 6.1 instructions posted, but if you don't wish to upgrade, or can't for whatever reason - you could contact me through the link above and I could add it to your existing site.

Joshua Pettit
Web Developer
www.ThreadsDev.net | www.JoshuaPettit.com