I am trying to open a file created in Mac InDesign 2., but only have
InDesign 1.5.2. on my Powerbook G4. When I try this, a dialog box
comes up and says, Cannot open file, InDesign may not support the file
format or a plug-in that supports the format may be missing.

You would need to open this file and Save As.. a version 1.x document.
(if InDesign 2 even has this feature).  The InDesign 2 document format
is designed to save the new features of InDesign 2, and is not
backward compatible.  Your 1.5.2 does not understand the new format
and thus complains.  If you can save it to an older format, your 1.5.2
should be able to open it.