When you boot for the first time, you need to have a boot loader running. Until you use either a usb stick or Boot123 disk, it will not understand the file system installed on the SSD
So, with either of the above, installed in the machine, with USB legacy enabled, start the mini. It will load DARWIN. It will give you a boot prompt. If you are using a DVD drive, enter 9f, if you are using the usb stick, enter 80, or 81.
Once your mini boots into OS X, run the DellEFI program, either from the usb stick, or the Boot123 disc, or download it directly to the machine. Select easy installation for standard installation. You will need to enter your password, or hit enter if you don't have one.
When it finishes running, restart your machine. I think it tells you you need to, I don't remember. Remove your USB stick if that is what you used to boot, or your DVD if that was what you used. You should see a grey screen with an apple logo and a spinning thing, and then it should boot to your desktop.