Thanks!
OK, got it to work. Kinda.
The problem was, apparently, that Project64 2.1 can't create cache files, so you have to first create them using an earlier version. That's all well and good, but the only plugin I got to work was glide64. The thing is, I want to use the Community Retexture Project pack. The pack requires a cache file larger than 300MB, which is the maximum glide64 is capable of (at least, it's the maximum value I can set the cache to).