I have exchanged data between Macs and PCs by having a Mac application (like Reunion) export its data in Gedcom format, and then having a PC application import it--again, telling it to expect incoming Gedcom format.
There are some details like using a diskette that is formatted for PCs.Macs can read and write PC disks, but PCs aren't so sophisticated, er, broad-minded.So be sure to start with a PC-formatted diskette...they all seem to come from the store that way now.
Of course, all of this assumes that the file you were given is in Gedcom format.You can check that by starting your word processor software (Word, WordPerfect, whatever) and then selecting the File Menu and Open.Select the file that you were given.A Gedcom file will look something like this on the screen, and you will know that you can import it into your PC genealogy software:
0HEAD 1 SOUR REUNION 1 DEST PAF 1 DATE 1 FILE etc.
On the other hand, if it doesn't look similar to that, or if the word processor whines, then probably the information is still in a Reunion database and hasn't been exported yet.You might be able to find a Mac genealogist in your own area (or by mail) who can export it for you.Honestly, it's pretty easy once you have a Gedcom file.