To access this dialog box in Learn mode:
Select the Point Only Mode and Find Nominals check boxes from the General tab of the Setup Options dialog box (Edit | Preferences | Setup).
Ensure that you have imported a CAD's solid model.
Select the Surface
mode icon 
from the Graphics
Modes toolbar.
Select the Override FindNoms menu option. The FindNoms Override dialog box appears.
With the FindNoms Override dialog box open, when you take a manual hit, PC-DMIS follows its normal logic to choose as accurate a nominal point as possible by using the most recently measured hit coordinates and approach vector.
You can then use the FindNoms Override dialog box to perform additional procedures allowing you to override the found nominal point.
More:
Procedure 1: Finding Nominal Points on the Previous Surface
Procedure 2: Finding Nominal Points on Priority Surfaces
Procedure 3: Finding Nominal Points on All CAD Surfaces