I presume that you are running a copy of the PMDX-Sample profile, or a profile that has the PMDX-SmartBOB plug-in enabled and selected as the motion device.
When you first display the "Motors" tab in the config dialog, the tuning parameter fields along the bottom of the window ("Counts per Unit", "Velocity", etc.) are grayed out. You must click on one of the four motors in the upper right ("Motor0", "Motor1", etc.) in order to enable the fields and allow you to edit them.
NOTE: Click on the text "Motor0", etc. and not on the check box. Clicking on the check box will check (enable) or un-check (disable) that motor, but will not enable the tuning param fields.
If this is not the problem you have been having, or if this doesn't work, let me know.
Bob
