I don't have the mission packs, so I'm just taking this information from sites I found online.
Go to the official KMQuake download site: http://www.markshan.com/knightmare
Download this file:
Official mission pack support (also needed for Pax Imperia):
Install that on top of KMQuake.
I don't use KMQuake, so I don't know if it will let you select the mission packs after starting the game. An option though is to create a batch file that should let you chose.
A batch file is just a text file with the extension .bat (instead of .txt). It's a plain text file.
I found this guide online:
I've found a solution, but it isn't an elegant one. Basically, create a quake2.bat file with the following text, and it should help you with playing the expansions. (put it in your quake 2 installation directory. You can create a desktop shortcut to the batch file if you want).
echo Quake II + expansions
echo 1: Quake II
echo 2: Mission Pack 1: The Reckoning
echo 3: Mission Pack 2: Ground Zero
set /p userinp=choose a number (1-3):
kmquake2.exe +set game %game%
Or, you can create separate batch files:
kmquake2.exe +set game xatrix
other batch file:
kmquake2.exe +set game rogue
Again, I haven't tested that but it should work.