Thread:Vorknkx/@comment-26005008-20150729053117/@comment-1496755-20150729115612

Aaaah, there's your problem - the mission pack has a separate executable. It doesn't work like Quake's add-ons. The original game's exe does not support the mission pack's features.

You have to run the expansion exe - its icon is a large red "X" with a "+" next to it. Its name should be something like "h2mp.exe".