The lkkbd driver has very good documentation in the long comment at the start of the file. It gives you the pinout, tells you how to hook up the keyboard, and tells you about the DEC document which describes the LK201 keyboard. (The DEC document is unavailable at that link but can be downloaded here. It's a technical manual for a VAX graphics board set which uses these keyboards.)
The keyboard uses +12V power and communicates via RS-423 serial communication at 4800 baud. This is compatible with standard RS-232 serial ports in terms of both protocol and voltage level. All that needs to be done is to hook it up to a serial port and provide +12V power. The connector is a plug similar to a phone handset connector. It can fit into a standard RJ11 phone jack, but because it is narrower it can fit in two different ways, one of which would make wrong connections.
Ubuntu comes with an already compiled lkkbd kernel module. To enable, the keyboard, all that's needed is something like:
sudo inputattach --daemon -lk /dev/ttyS0
This will load the module and make the keyboard usable, leaving inputattach running as a daemon. The last part of the command specifies the serial port; change it if necessary.
The LK401 is pretty good. The tactile feel is better than the LK201, and I can type quite fast with it. The driver is stable. It seems all keys except Select work. There are no X11 keysyms associated with F19 and F20. Most of the extra keys are reported as XF86Launch