To make a Custom Panel usable, you must activate it. Activating a Custom Panel changes it from an editable state (designer) to an operative state (activated).
NOTE: Make sure to activate a Custom Panel again after any changes have been made to it. Otherwise, it will not reflect the most recent editing.
There are two ways to activate a Custom Panel:
From the panel menu, press the F5 key
Press "Launch Current" in the Panels Ribbon