Sometimes you may want to open a 'window' in your solder mask/resist to allow something like a copper shape to be available as a conducting surface. Plotting solder masks will automatically deal with generating windows for the necessary pads/vias, but for copper you will need to make the windows yourself.
Assuming you already have a copper shape (either in the design or in a footprint) that you want to keep clear of resist, here is all you need to do:
Select the copper shape
Right-click and 'Cancel Move'
Select the new copper shape, right-click and 'Change Shape Type'
Change the type to 'Doc Shape'
Right-click and 'Change Layer' to change to the required layer
You should now have a shape on your resist layer that corresponds to the copper shape. If you want to slightly 'oversize' your mask shape:
Select the shape
Right-click and 'Resize Shape'
Enter the amount by which you want the shape to be grown