Thanks! That did it.The page is here:
The Thread Rating System is an add-on that allows you to rate the threads within your forum.xentr.net
If you're not seeing a Buy option, it's probably either because you haven't yet got admin approval or you haven't verified your XF license. You can do that here:
And if you've already done that at some point in the past, well, for some reason they require you to do it every couple of months (three?), for whatever reason.