you need to launch a new instance of PI from the console like so:
!!$PXI_COREDIR/PixInsight -n
note that this command differs from previous replies on this topic in the forum... i think this is because Juan had to rejigger how the PI binary is packaged to be compatible with apple's new application security guidelines. this may have started in OSX 10.10 or 10.11, not sure. can't even remember what apple calls this new stuff (signed binaries, etc.)
rob