JJ2+ is basically a patch for JJ2 that adds a heap of stuff and fixes a bunch of bugs. Most of it is for multiplayer, but there's been some single player stuff added recently, so it's worth getting. Note that it only works with normal 1.23 or TSF, so if you have one of the Christmas versions of JJ2 or the LK Avalon version (which it seems you have) then you won't be able to use it. To be honest I have very little idea of what the LK Avalon version is, but it doesn't work with JJ2+, or some other useful programs, so you'll have to find a way to get a normal version of JJ2 if you want to use Plus.