You have to do a fetch on the ArrayController in applicationDidFinishLaunching:
in order to see your Entities there. Try doing this before accessing anything from the ArrayController:
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
NSError *error;
[arrayManager fetchWithRequest:nil merge:NO error:&error];
Entity *ent = arrayManager.arrangedObjects[0];
}