The control board in the printer connects via a USB to serial bridge. To the computer connecting to it, it looks like a serial port. You see this on a PC as well, Windows uses COM ports, Linux/OSX use /dev/tty* devices, etc… I haven’t set up a Pi to control my Mini yet, but that’s the basic idea.
There is usually an option to auto-detect the serial port, that should find the printer. If not, try to find it the long way…
Log into the command line on the Pi using SSH or whatever.
With the printer disconnected, do “ls /dev/tty*”. There will be a lot, but you can scroll up in the terminal window if on SSH. If you are using the Pi connected to a monitor/keyboard Shift-PgUp should work.
Now plug the printer in again, wait a bit, maybe even use “lsusb” to make sure the system sees it and has a chance to set it up. Now do the same “ls /dev/tty*” again. Look for a new entry. It’s probably /dev/tty.usbmodem with some extra stuff on the end. That’s your serial port.
You can also plug the printer in and do “dmesg”. Look toward the end, there should be a log entry for the printer attaching. It might not show the device name though. It depends on how the logging was done for that driver.