Those would be WordPerfect files. I'm surprised that the Microsoft Word version for Mac can't seem to open them, but a quick Google confirmed this.
Oddly enough, it doesn't seem that iWork or OpenOffice can either.
Thankfully, it appears that another popular Open Source Word Processor, AbiWord, can.
Download AbiWord for Mac from here:
Download AbiWord for Mac, AbiWord for Mac 2.6.4 Download
I would then recommend doing a "Save As" and converting to a more standard format.