So 1st, does the board work? as does it output characters when you type on it?
-If yes, then the problem is either likely software side (windows drivers/config, OSX/linux firmware/config) or that there is something wrong with the connections in the board, tbh my lexmark M is the terminal kind and does not have LEDs so i am out of my depths here, or it could be that the controller is damaged, but that is rather unlikely.
-If partially (the keyboard works sometimes randomly stops, outputs long strings of the last char you pressed), then i would expect that either your usb port is not supplying enough current (if you are on a hub for example) or that your converter is not M compatible and crashes (had that with a belkin ps2 to usb, a dock with integrated converter and a random no brand in wire converter). 1st step if you are on a hub is to plug on the pc directly, 2nd is to change the converter.
-If no, then the controller is not initializing, and that could come from a lot of things, from a dead board, dead converter, windows 8 having ps/2 capabilities disabled by default, pc/macs with incomplete USB compliance cutting power.
a lot of this comes from power, the M is a rather power hungry board, especially the old ones, so it can be rather hard to drive.