Once the runtime files for Quake II .NET have been installed, you need just one more file, pak0.pak, to run the .NET version. The PAK file contains id Software’s copy written 3-D models and images and they’d prefer that you get that file from them:

All of the Q2 data files remain copyrighted and licensed under the original terms, so you cannot redistribute data from the original game… -John Carmack, id Software, from the readme.txt in the GPL’d source code

Therefore, you need to get the PAK file from the official Quake II demo. Here’s what you need to do:

1. Download and open the Quake II demo from ftp://ftp.idsoftware.com/idstuff/quake2/q2-314-demo-x86.exe. This unpacks files to your system (default folder is c:\windows\desktop\Quake2 Demo).
   
2. Copy the pak0.pak file from the Quake2 Demo\Install\Data\baseq2 folder to the %ProgramFiles%\Quake II .NET\managed\baseq2 and %ProgramFiles%\Quake II.NET\native\baseq2 folders. This file is big - about 48MB and you’re making two copies. If you uninstall Quake II .NET, you have to remove these two copies by hand.
   
3. Run the managed or native version by clicking the shortcut in the Start menu.