Drill a large enough hole (3/4" - 7/8"??) in the top of your kegerator while being certain not to hit anything crucial. The hole should be large enough so that the pan will sit flush on the top surface. Not larger than the washer.
Silicone, with a small bead, the outside edge of the
drain's bottom surface, set the pan and then another bead around the outside edge. Place the washer and nut on the stem and tighten.
The drain stem may be shortened if it is in the way. Not too short as you will want to clamp a 3/4" ID hose onto this and extend hose into an empty half gallon milk jug. You might want to route this tube left or right and then fasten to the side wall, then into the jug.