Bonjour lol101,
This is very simple. In XSI, open a browser (or hit 5). Browse to where the .preset file is, select it, drag and drop it on the object (you'll notice that a plus sign is added to the pointer, indicating that you can drop the file on this object), in the viewport or in the Explorer. Simple as that.
Or: Assign any material. When the ppg pops-up, click on the Edit Parent arrow. Now you're in the material properties. Click on load, and load the .preset file.
Hope this helps
Salutations - Cheers
Bernard Lebel