I too am having trouble getting a reader (MCR12) to talk to an Arduino Uno. I've made my own four wire cable and have wired it up like this:
Scanner pin 1 to AX Gnd
Scanner pin 2 to AX +5V
Scanner pin 7 to AX Data pin
Scanner pin 8 to AX Clock pin
Here's the scanner pin chart from the manual:
I've tried multiple different variations of the PS2Keyboard sketch and don't get any data from the scanner when I open serial monitor. All I see are "keyboard test" or similar. The scanner has power and scans fine, but nothing is being received by the AX.
I've also tried swapping round the data and clock wires going into the AX, as well as inserting pull up resistors. I've also tried different pins depending on the sketch (usually 5 & 8 or 2 & 3).
Does anyone have any idea why this isn't working?