It looks like you're getting the error by double clicking the icon in Windows 10. You can't do that. These are old, DOS based programs.
You have to download DOSBox: http://www.dosbox.com/
The 32 bit installer should work fine on Windows 10.
Install it, and then run it. You'll start on the command line, Z: Drive.
You'll need to mount your Folder in DOSBox, so that the program will see it as a drive.
Something like: mount C C:\realms\unlimi~1\If
you have UAShell installed in C:\realms\Unlimited Adventures\
Once you have it mounted, and that can be the tricky part, switch over to your mounted C drive.
Just type: C:
The drive will change from Z: to C: now.
When that happens, type: uashell
You'll be able to apply a design now.
Keep in mind that at this point you're in DOS. You can type DIR to see what's in the directory, and CD for change directory. Maybe CD Temp, if you had to go deeper.
Hope this helps.
Mounting is probably the hardest part, so if you have any more questions just ask.