From https://home.hccnet.nl/h.g.muller/download.html:

> To play you must enter moves in algebraic notation, and to make the computer do a move you should press an extra enter. You can alternate sides whenever you want, the program accepts or plays moves for the side whose turn it is to move. After having done a move, be it yours or his own, micro-Max prints a board with the new position. To quit type control-c.

The instructions say you must enter "algebraic notation," but I couldn't figure it out until I took a peek at the source code to see how it was trying to parse input.

You always just enter source square to destination square. So to play an opening move 1. d4, you would type `d2d4` (move piece on d2 to d4) and press enter. Then if you want micromax to play the next move for black, just press enter again. It'll think for quite a while and draw the board with the new position. To play 2. Bf4, you'd then type 'c1f4', and so on.

(This appears to be "long algebraic notation" and is apparently common in chess engines, now that I look it up on Wikipedia.)