Hi Devina,
If the field is extended from another table you should be able to dot-walk to it, in the UI policy action you should be able to navigate to the extended table via the tree picker in the field name element.
If it's not available then you can create the UI policy on the table where the field is extended from and tick the 'inherit' box, this will cause all extended tables using that field to inherit the policy, you would probably then need to define some conditions to ensure it only applies on the table you want it to.
If you go down the dictionary override route, you need to right click on the field and select Configure Dictionary. In the advanced view you will see a tab called dictionary overrides. Create a new one, set the table to the one on which you want your field to be mandatory and then tick 'override mandatory' and then tick 'mandatory' when it appears.
