I am having the same issue, currently running OS X El Capitan (OS version 10.11.6). I have tried uninstalling and re-installing and resetting my computer, along with double checking the software wasn't actually running via attempting a force close. If it helps at all, I have attached this screenshot showing the flag, my force close window, and my applications folder with the pokemmo client in it.
Thanks for any help! Plz fix I wanna play plat :'(