OK, got it to work.

1) Put the application and all of the other files on the distribution disk into a folder named Celestia (not Celestia ƒ, etc.) on your disk.

2) Make the change indicated in the previous postings to uncomment the IgnoreGLExtensions line (remove the # from the beginning of the line that says: # IgnoreGLExtensions [ "GL_ARB_vertex_program" ]).

3) Make sure you do not have anything in your DYLD_LIBRARY_PATH that refers to a directory that contains any other version of libpng.dylib (I had a reference to /sw/lib for other programs that I use that require newer versions of this library, so just make sure this is not the case 00- use echo $PATH from within the terminal to find out, and also check your ~/.MacOSX/environment.plist file to make sure there are not any such conflicting path references in it.)

Then it works fine, and goes through the demo, etc. without problems.