Thanks to Leo Nathan's help, it works now.
self.masterPopOverController = [[UIPopoverController alloc] initWithContentViewController:referenceLibraryVC];
[self.masterPopOverController presentPopoverFromRect:[sender frame] inView:[sender superview] permittedArrowDirections:UIPopoverArrowDirectionAny animated:YES];
I didn't connect it correctly in the XIB file and it wasn't implemented in the correct place in the IBAction for ButtonPress