The MK-90 cartridge can be read and written with a simple circuit connected to the parallel port. The required +5V supply can be drawn from the keyboard, PS2 mouse, or USB port.
For a similar device controlled through the USB port, please refer to the SMPReaderUSB designed by Ilya Danilov. It can also be built by repurposing an USBasp programmer board - project page, additional hints.
Any general purpose small signal npn transistors can be used, for example 2N3904, BC547.
The control software is as simple as the circuit.
mk90prog.zip - file size: 18kB, source and executable
The program won't run under Windows 2000, NT, XP, because these systems don't allow direct hardware access from user programs.
Insert the cartridge, apply +5V then run one of these programs: