You can start mission packs with AppleScript.
Quit Quake.
Copy the folder of the mission pack to your "Quake" folder. The "Quake" folder is the folder that holds the "id1" folder.
Open the application "Terminal" which is part of the MacOS X installation.
Change to the directory that holds the desired Quake application. For example: "cd /Applications/Games/".
Start the desired Quake application by appending the command line parameter "-hipnotic" or "-rogue". For example: "./Quake.app/Contents/MacOS/Quake -hipnotic".
Quit Quake.
Copy the folder of the mission pack to your "Quake" folder. The "Quake" folder is the folder that holds the "id1" folder.
Open the application "Terminal" which is part of the MacOS X installation.
Change to the directory that holds the desired Quake application. For example: "cd /Applications/Games/".
Start the desired Quake application by appending the command line parameter "-game" followed by the name of your mission pack.