I saw your posting on AppleFritter, but I will echo my reply here, and hopefully someone else knows a better or easier way (not that this is bad at all).
Use F4 to mount the disk image (sounds like you're doing this fine). The IIGS will only boot from the first disk in a slot, so S5D1 or S6D1 would work but S6D2 would not for example.
Restart the emulator, if it doesn't boot properly, most likely you will have a "*" prompt (monitor) or "]" prompt (basic).
If you are familiar with using PR#5 or PR#6 to boot a slot (from BASIC), that's really all you need to do. If you are at the monitor "*" prompt, type "E000R" to get to BASIC, and then PR#5 to boot Slot 5 disk 1. It should work for you.