1.) go to Setup; Setup parameters; Planes- and define what layer(s) you want to pour copper onto along with what nets you want to have on the plane (set the type Neg for an internal plane, Pos for top or bottom routing layer).
2.) go to Route; Planes; Planes processor- this is where you will define your plane outline ,clearances and auto generated connections to the planes (You can select "route boarder" to fill in the internal area, or you will have to draw a plane shape with the edit; place;plane shape menu)
Read the Help files for a better understanding of these procedures and experiment with the Planes Processor button until you get what you want.
Good Luck