If you need to find out what folder iPhoto is keeping your photos in, you can go to the File menu in iPhoto and select "Reveal in Finder" - This will open up Finder to that folder. You can open up a Photo Mechanic contact sheet for that same folder.
If you do this regularly, you can create a "Favorite" in Photo Mechanic for the root folder that iPhoto uses so that you're not having to always browse deep into your folders to find them.
-Mick