Hmmmmmm...tried adding it to my 6.5 board and hacking the four original files was no problem whatsoever.
I uploaded the four modified original files (public_forum_page.pl, public_topic_page.pl, ultimatebb.cgi, vars_wordlets_mods.cgi), the one new cgi file (ubb_rate_topic.cgi), the rating star GIFS to the UBB folder and I added the "thread_rating" folder (plus the recommended index.html) and properly chmoded all the files (cgi=755, vars=777, .pl=644 uploaded in ASCII and image GIFS uploaded in Binary and not chmoded)
Although I can see the new wordlets in the control panel, I can't see it on the board anyplace.
I looked at your Demo link.
https://www.ubbdev.com/db/cgi/ultimatebb.cgi And I can't seem to see it there either.
So either I goofed up somewhere, I'm blind and can't see the modification or this doesn't work with 6.5
