Adding Masters to Wireframes
There are a couple ways to add masters to a wireframe.
Drag and Drop
The primary method for adding masters is to drag and drop master from the Masters pane onto the wireframe pane.
Copying and Pasting from Axure RP
Masters can be pasted onto the wireframe pane. Masters copied from an Axure RP wireframe pane can be pasted onto the pane.
If you copy a master from one design file A to another file B by having two instances of Axure RP open, the master in A will the added to B. If you had previously pasted or imported the master from A to B, the master will be pasted but will not override or sync the master in B. To sync the master in B with the master in A, you need to use the import feature.
If Wireframe->Mask Masters is checked. The widgets in the master will be masked to appear different from other widgets on the wireframe.