To add a period to your planbook, click on "Planbooks" in your blue navigation bar.   

Then, click "Edit" for the planbook you are working in.

Here, you can select the number of periods for your planbook.

Note: Added periods will appear at the END of your existing periods.  If you decrease the number of periods, it will take them from the bottom up.  For information on how to move your periods, see our article on reordering periods.