In Cakewalk Pro Audio 9, you cannot dither from 24 bits to 16 bits when you mixdown your audio. This conversion must be done beforehand. Go to Tools - Change Audio Format, choose 16 bit, select dither, and hit OK. This will convert all audio files to 16 bit. The best way to change from 24 bit to 16 bit is to mixdown into 24 bits and use an external program which will perform the dithering such as a 2-track editoring or mastering software.