I have the collection (bought from Walmart recently) and Kotor Tool. The latter will let me mod 'Knights' but not The Sith Lords. Is that because I installed the game after Kotor Tool, or what? Many thanks for your answers.

In KotOR tool, go into the paths manager and make sure that TSL's path is in the relevant field. If not, either hit the button that looks it up in the registry or copy and paste it in manually.

I had this very same issue, though in the end I had to go into the registry myself and add a Registry Key for TSL manually, since the installer hadn't made one fir sone reason. That might have just been a problem with my computer though, so try the first two suggestions I made first. :)