I would imagine you just use left/right Up/Down arrow keys for jogging X and Y and Pg Up/Pg Dn for Z.
The voltages could be right if they are constantly changing if it is the Step pins as they will be pulses so a voltmeter will only read the average and how much that is will depend on the speed of the jog/motion. The Dir pins however should either be 5v or 0v, your 4.3V is close enough to 5 :0
Its best to start at the port with the cable disconnected then if that checks out put the cable in to the port and again check at the other end, that way you know what is coming out the port and then getting to the breakout, testing while running at the breakout output will just tell you if there is a problem somewhere but you wont know where.
Hood