Yeah, I know about RigelEngine. I must have stumbled across it back in January 2018 or so when I was giving some feedback on
Cosmo-Engine and helping the author fix some sound bugs in Cosmo-Engine. The author of RigelEngine was (and still is) watching the Cosmo-Engine project on GitHub and his avatar is obviously inspired by Duke Nukem II, so I checked his projects and found RigelEngine.
Anyway, back on topic:
The very first thing you're supposed to do when you start plaing around with the editor is extract all of the contents of NUKEM2.CMP to gain access to all the levels, tilesets, songs etc. The editor uses the standard open file and close file dialogs from Windows and those obviously don't allow you to open files that are hidden inside other files and aren't visible for Windows.
The CZone files should show up in the tileset selection window when you create a new level or edit the settings of the current level. This will also list files from within NUKEM2.CMP even if you haven't extracted the files. But for the CZone editor, the CZone files have to be extracted from the CMP file before you can open them.
The music testing feature requires external tools. Music playback needs an OPL (AdLib) emulator and I wasn't able to port any existing OPL emulation code to programming language this tool is written in. The most recent version of
K1n9_Duk3's Enormous Tool supports my
IMF to WAV Converter as well as
Gerstrong's IMF Player (you can find downloads for both of them on
my IMF tools page). The download for
K1n9_Duk3's Enormous Tool should already include a zip archive of my
IMF to WAV Converter, but it's an old version.
Extract the IMF tools into the same folder as Duke2Edit.exe and it should work right away. You might also want to check Duke2Edit.ini and make sure the "path" settings in the [IMFCONVERTER] and [IMFPLAYER] sections are correct. The way the level editor interfaces with these external programs is a bit of an ugly hack, but I just tested it under Windows 10 and it worked fine.
All of this was already explained in the readme.txt file that comes with the editor, but it seems that nobody reads readme files anymore.