Use the **Copper Area** tool:  Assign the netname of the Copper Area to the same netname as you have used for your ground net in the schematic (and therefore passed into the PCB). Note that the netname assigned to the Copper Area must be one of the netnames used in the schematic. If the netname assigned to the Copper Area does not exist in the schematic (and therefore not in the PCB either) then the copper area will not show any fill.

You can get a copper area to fill even if the nets name assigned to it does not exist in the PCB by setting "Keep islands" to YES but having a floating copper area is not recommended as it can cause problems with crosstalk.