SAFE METHOD: This method doesn't require to pry open the white plastic cover, to get the replacement feet in.
Prerequisites: toothpick / patience
1. Lay the trackpad on its back so you can see the rubber feet hole. We'll leverage the fact, that when you click the trackpad, the gap inside the hole becomes a touch wider.
2. The rubber foot has a small solid piece of plastic in the middle. Put the rubber feet above the center of the hole.
3. Make sure you get the click noise when you apply a bit of pressure in the center of the replacement foot. This widens the gap between the plastic cover and the metal.
4. Use one finger to keep the pressure on the rubber foot center, so that the gap stays widened the whole time.
5.Use the toothpick with your other hand to carefully push the ends of the thin rubber underneath the white plastic cover. The rubber should almost slide underneath it by itself with a little bit of pushing from the toothpick. This might take several tries and patience, but it's a risk free method!
WHERE TO BUY: Just search on ebay for “Magic Trackpad Touchpad A1339 Bottom Rubber Feet Foot”, you'll find some vendors from China.