I am trying to create ePub documents with CS4 Mac (OS X10.5),
Think hard about an upgrade to CS5.5 - I am not an ePub maker at all, but by all accounts it's mind-bogglingly superior for ePub production, if you know what you are doing.
Also, does anyone know if XML is the same as XHTML? ePub calls for XHTML, but evidently InDesign doesn't create that, at least I haven't found a way to do so.
They're not the same at all; XHTML is a subset of XML, in the same way that e.g. dogs are a subset of "mammal." You can export ePub from CS4 by going to File -> Export for Digital Editions, and that will spit out something that can supposedly be made into a usable ePub by monkeying around with the code inside. However, once again, the CS4 support for ePub production is extremely minimal, so even though I don't make any ePubs at all I feel pretty confident in telling you "Don't bother; upgrade to CS5.5 if you want to make ePubs."
I am trying to create ePub documents with CS4 Mac (OS X10.5), but every time I try to export to XML, it creates a blank page.
... and the above is why. InDesign can import and export XML, but these have basically nothing to do with ePub export, so I won't go into any details about how to get XML export from ID, because it sounds like you don't actually need it.
Which leads me back to "... if you know what you are doing." I think that, even with 5.5, you need some web development skills to make ePub, even with InDesign. For example, I think that you need to know more about how to edit the CSS generated by InDesign, or perhaps even write your own from the ground up. Depending on how much you like to learn about new technologies, you may want to invest in a book or three, or consider investing in a contractor to hammer out the ePub on your behalf. If none of the ePub-savvy forums regulars post in your thread, I'll go find one of the threads where they've recommended books and post a link for you.
Edit: I think I was too generous in describing the ePub capabilities of CS4.