Are you launching the installer from the cdrom directory? i.e.:
cd /path/to/cdrom
wine setup.exe

???

If so, that's probably the reason why. If you're "in" the directory that you are trying to umount, it won't umount.

Instead, run the installer like this: (from home directory or any dir other than one on the cdrom)
wine /path/to/cdrom/setup.exe

You should be able to umount the cdrom if you do this...

Good luck!