No worries mate
You should only need the .pk3
file. Put it into:
C:\Program Files\LucasArts\Star Wars JK II Jedi Outcast\GameData\base
(assuming yours is the auto install-path).
Start a single player game as normal, then bring down the console - Shift + `
that's the key under your Esc and next to 1.
Then type map demo
and press enter/return.
The demo map will load (no snow though
It might be an idea to save the game as soon as the map loads, then you can start the map from the savegame in the future, and not have to do the console thing every time.