The luggage drop off area will actually be pretty easy to find. You'll see the luggage porters, luggage racks, and luggage waiting to be loaded toward the western part of the pier area.
Please keep in mind that your taxi may be directed by port personnel to drop you off at the Disney Cruise area in the eastern part of the pier. (At least this is what happened to us.) This would require you to walk yourselves to the RCCL area. (Don't fret, though, it's pretty close)
For what it's worth, the embarkation process in San Juan is, by, far, the quickest we have experienced. From Taxi dropoff>walk to luggage dropoff>check-in>boarding took less than 15 minutes!!!
