First you'll want to make sure that your operating system is treating the projector display as a secondary display rather than a mirrored display.
On Windows you will find this setting in the Control Panel. Be sure Extend these displays is selected.
On Mac you'll find this setting in System Preferences. Be sure Mirror Displays is unchecked.
When you go On Air Proclaim will automatically attempt to open the presentation window on the secondary display, falling back to the primary (only) display. Additional information on configuring display outputs can be found here.
If you're on a Mac and your secondary monitor is not displaying when you Identify displays in the Proclaim Settings menu, the app may be assigned to a particular desktop. To fix this, right-click on the Proclaim app icon in the Dock, select Options, and None from the list.