NSTANOSHECK  Too bad for you, Ubuntu canceled the PPC version of their OS. Personally I preferred YDL 4.1 to Ubuntu but Different strokes for different folks I guess.  
(Version 5.0.1)

KARX11ERX  If unzipping the textures, be aware that they are already zipped with a subfolder 'textures', so unzipping to e.g. c:\descent\textures will put them in c:\descent\textures\textures (WinXP type folder names, but I guess you get the point). Turn off folder usage when unzipping or unzip to your main Descent folder.  
(Version 1.6.130)

SIMX  Custom levels are never in .exe format. There are usually two files (a .hog and a .mn2) for custom levels and they should work fine. Descent 1 levels should also work fine -- if they do not, please post a bug report at http://www.descentbb.net/ in the D2X-XL forum.  
(Version 1.6.122)

JKXZDF  Well... when I go to the "3rd party singleplayer" section of descent2.de there are several levels that are an exe. Some of which are "encounter at farpoint 2," "sequel of the entropy experiment," and "obsidian."  
(Version 1.6.122)

SIMX  The .exe files should be expanded by Stuffit Expander; the .exe files are not the level files themselves. I tested out the file from Encounter at FarPoint 2 -- once expanded with Stuffit, the required eaf2.hog and eaf2.mn2 files can be placed in the required folder. I tested the map, and it works with D2X-XL on the Mac.  
(Version 1.6.122)

KARX11ERX  Are you using the PC Descent 1 data?

All missions on my Descent 2 site are contained in compressed archives. Usually these are zip files. Some missions are however self-extracting archives and hence MS Windows executables. That's how they were presented by their developers.

Please follow the directions given by simX, the MAC developer of this application, on how to install them.  
(Version 1.6.122)

JKXZDF  I had no idea I could use exe files with stuffit. I uncompressed it and the level works fine now. Another thing... will those other high-rez textures work on Mac soon? I know the high-rez pyro works since I use it but the others like the doors and walls don't seem to work yet.  
(Version 1.6.130)

SIMX  Mmhmm, Stuffit has been able to make and expand self-extracting archives for a while now.

As for the hi-res textures, yes, those work already too. :) You need to put a "-hires_textures 1" line in your d2x.ini file. Also, go to [Configure --> Miscellaneous] and make sure "Show Expert Options" is on. Then go to [Configure --> Render Options --> Advanced Render Options] and make sure the render and texture quality are set to high.  
(Version 1.6.130)

JKXZDF  I guess my copy of d2x-xl 1.6.130 is just lazy since it still doesn't show the high-rez textures... excluding the pyro-gl one.  
(Version 1.6.130)

SIMX  Have you dumped all the textures in the Data folder, or have you created a subfolder called Textures in the data folder and dumped them in there?  
(Version 1.6.130)

JKXZDF  I created a textures subfolder.  
(Version 1.6.130)