OK, first the guy at Best Buy is an idiot. (Feel free to tell him I said that.)
If the motherboard has an IDE connector on it, then your job is easy. Set the IDE drive for Master, and plug it in.
You don’t need to change anything on the SATA drive as they are not being plugged into each other. You only need to set a drive to slave if that drive will be on the same cable as a drive which is set to master. As IDE and SATA drives can’t be plugged into each other, there’s no problem there.
Your CD/DVD is probably IDE (could be SATA) so you can plug the IDE drive into the same cable as the CD/DVD drive just set one for master and one for slave.