In CADSTAR:
The layer needs to be an electrical layer, draw a template around the board (either in Design Editor or PREditor) and in the template properties select the net the copper will be connected to. Use the help file to explain further info about the use of the template.
Then either in the Embedded Router or PREditor select the template and pour the copper.
(ER has an icon, PREd RMB and "repour).
If the pour does not look right select the template properties, adjust the settings and repour.