The motor tuning window has nothing to do with the screenset.
It's a somewhat common issue, which I think requires a re-install of Mach3.
I just searched it, and it appears that if you rename the Mach3 folder, or don;t do a proper install, the motor tuning window will not work
The problem is that the NTGRAPH.OCX file that the motor tuning window uses is a registered ocx file. If you rename or move the installation, then Windows can't find it. You can manually reregister it by clicking the Windows start button, then Run and enter:
regsvr32 {full install path}\ntgraph.ocx
For example for a standard install it would be
regsvr32 c:\mach3\ntgraph.ocx
Generally, a screenset should have no effect on Mach3 operation. If something is not working after changing screens, it's usually due to them not being installed properly.
You mentioned homing not working in your other thread. Homing is handled by a macro, usually saved in the button. The new screen may use a different homing macro.