I don't have a Mac, but I think it should work just like it does on Windows and Linux:
"Tools" menu --> "Options" (or maybe "Options" will be under "Edit")
On the left, under "Load/Save", choose "General"
On the bottom, under "Document Type", choose "Text Document" from the list on the left and "Microsoft Word 2003/XP/whatever" on the right.
Do the same for "Spreadsheet" and "Presentation" document types.