What is required before maps/skins/weapons packs...

[Gweth]

First, obviously, install the game.

Then install the 1.01 patch: http://jediknight3.filefront.com/file/Jedi_Knight_Academy_Patch_101;20459

Followed by the JAplus(JA+ 2.3) mod: http://www.lucasfiles.com/index.php?s=&action=file&id=1074

In that order, before anything else, otherwise problems may be abundant for you.