Sorry no one has responded. I'm not an editor expert, but here's what I know:
1. You need to make sure you are working with version 1.21g of Civ3. If you have an earlier version then get the 1.21g updater from the MacSoft support site.
2. You need to have a copy of the Civ3Edit program file, version 1.32.1, and place that in your Civ3 application folder, at the top level, NOT in it's own folder.
3. If you want to add units and other items you need to modify the Civ3Edit software using the NIB file that displays the Add/Delete buttons. There's a brief description of how to add the NB file in post 106
here and acouple of posts further down there's a link for the NIB file.
4. Once you have that lot operaitonal you need to go to the Creation and Customisation forum and seek out the tutorials on how to add things. Units require additions to the .BIC file using the editor, and they also have to have graphics files and additional text entries in the pediaIcons.txt and civilopedia.txt files. It's beyond the scope of this post and my feeble brain to provide all that information.
5. One point to remember: If you are editing text files then ensure that you use a text editor that preserves DOS line endings - CR/LF - otherwise Civ3 crashes comprehensively. There's a modified TextEdit link in my signature that allows you to specify the line endings when you save a file.