You can't replace it because it is an integral part of the condenser. You can replace dessicant bag but you shouldn't have to unless your system was open for an extended period. Did your mechanic quickly and carefully plug the refrigerant lines while he was installing the new compressor?
I got the dryer as part of the kit, that is the dessicant bag. Do you know how to replace it? The compressor warranty says it is covered if you replace the dryer.