The seat on the Concept 2 Model D indoor rower is tested and designed to support both the rower’s weight and allow adequate blood circulation.
If you are rowing and feeling pain in your butt then the best thing to do is experiment with padding the seat. Try with a little folded towel, different shorts, a piece of bubble wrap or a special seat pad. If you are feeling numb during your rows try reduce the padding. You should also try adjusting the height of the footrests to test if it helps. Stretching and proper warm up might also help feeling more comfortable on the seat.
Leaning too far back at the finish of the stroke and you may be increasing pressure on your tailbone. Sliding some inches forward or backward on the seat during your workout can relieve the pressure on the problem area. Over time your rowing technique will be refined and you will get used to the seat.