When a resignation is offered a white square will appear on the board area. The square has a number 1, 2 or 3. A square with a 1, means that a single game resignation is offered, a 2 that a gammon resignation is offered, and a 3, means that a backgammon is offered. If you're using a 3D board, the resignation will be a white flag instead of a square.
To accept the resignation click on the numbered square. If you want to decline the resignation, you right click instead. You can also use the same toolbar buttons as for accepting or declining a double. There is also a menu option for accepting and declining resignations. Click Game->Agree to resignation from the menu, to accept an offered resignation. Click Game->Decline resignation from the menu, to decline an offered resignation.