Our 5040 tally printer is not being detected by a computer

I would guess that the printer is set to use the Serial Interface. I think this is the default interface setting on the 5040, hence the USB and parallel interface will be disabled.
You can change which interface is active from within the printer menu, once you have enabled the USB interface the PC should then detect the printer when you plug it in.

Details on entering the Menu on the 5040 can be found on the Dascom web site; http://dascom.firma.cc/support2/technical_notes/serial_matrix/5040/5040_tn004.htm