Please clarify what you mean by 'custom extension'?
I was trying to make an extension with Configurator 4 to make my own brush selection panel. By making the actions in Photoshop and then applying them to different action windows in Configurator, then exporting them into .zxp.
Ok Sarah and which version of Extension Manager do you have installed?
I may be wrong, but I think Configurator 4 might not work with PhotoShop CC (depends on the exact version, I think).
Configurator was phased out about a year ago.