Hello,
The are 2 types of planes you can build. Positive or negative.
Both require you to set up the layer definition.
On your toolbar go to setup then layer definition. Here a menu will pop up. Select the layer you desire the plane to be on. For a negative plane you will select the CAM Plane button. For a positive plane you will select the split/mixed button. And finally, you need to assign nets to the plane layer you just set up. Simply select the Assign nets box and your netlist will pop up. Here you can add the nets you want on this plane to the plane layer.
Close the box and go back to your design. Now you can build your planes.
Hope this is helpfull!
Eda