Again, to make cels, the only Mac tool available to you is CelMaker. CelMaker has a few little quirks that you want to keep in mind.
First of all, you have to make sure your cels are in the correct format. If you are making your cels in Photoshop, you need to save them as Pict Resource files (and make sure to name them with a maximum of 8 letters [not truely necessary, but if you want it to go on the BKP do it so more people can read it], and without the .cel file extension). If you are making your cels in Graphic Converter, you need to save them as StartupScreen files (again, follow the rules for naming, it will save you time and energy later). CelMaker will only read the files if they are save in this format (it is actually the same format, but the programs call it differently).
Note: Photoshop, at least my version, when I save a file as a Pict Resource, and there are 4 colors or less, almost always puts a white line vertically down the right side, often one pixel away from the edge. To avoid this problem, make sure your color table has at least 4 colors, an easy way is to edit the Color Table and just add an extra color when necessary.
Then we get to the real little CelMaker "quirk." The weirdest thing CelMaker does, and the most annoying, is the fact you have to make sure they have an even number for the width, or it throws in a line of garbage (just add a line of the transparent color down the right when you have to).
To actually use CelMaker to make cels, open CelMaker, choose your palette, and under the File menu choose Process Pict, and choose a cel. Repeat for all your cels. Its annoying and can take forever, but that's the way it works.