MUI is available directly from the Deneb card.
Set the jumper on the Deneb for "RES" (it is marked on the card) and when your system boots there will be a draw full of install files. Install and extract the files and you'll end up with a MUI installer, FAT file type and all sorts of other goodies.
One thing you may have to do is track down an updated "INSTALLER" command (from Aminet) to replace the older version on the WB3.0/3.1 disks which will fail.
I'm trying to understand how you installed the Deneb software without installing MUI.
MUI is not included in the installer image. You can perfectly use Poseidon without MUI, but you will not be able to configure things (or get popups).
To use Poseidon without MUI, use
AddUSBHardware DEVS:USBHardware/deneb*usb.device (where * is either z2 or z3pio or z3dma according to your configuration).
AddUSBClasses
SYS:Prefs/Trident NOGUI SAVE
After that, you can just use ENVARC:PsdStackloader to start the stack. Note that the above steps are also done during the first installation (though it is a bit tricky there, because when you are installing from the Rescue image, you are always in Z2 mode).
As mentioned in the other posting, you the mass storage class will by default attempt to use AmiCDFS as a CD file system and FAT95 for FAT formatted media. But you can of course use the included UMSD mountlist as a template to mount whatever you like.
To see what's happening use
PsdErrorlog
for a list of errors and messages.