Can't read PageMaker 6.5 files with InDesign CS3

I have a lot of old PageMaker 6.5 files that need to be upgraded to InDesign.

When I try to open them in InDesign CS3, I get an error that says:

Cannot open the file "XYZ". Adobe InDesign may not support the file format, a plug-in that supports the file format may be missing, or the file may be open in another application.

I get this error on a Mac Pro G5 (PowerPC), running OS X 10.5.8.

What's interesting is that when I try to open the same files on other computers on the network, it works just fine.  The other machines are all Intel Macs, running OS X 10.6.8 or greater.  The files are on a Mac Server.

Can this problem be fixed on the Mac G5, maybe by installing a plugin, or is there no hope?

Thanks!