Apple Remote Desktop Admin blocked by OS X Yosemite Firewall
Quit ARD if running, and go to [your home]/Library/Containers. Move the folder com.apple.RemoteDesktop to your desktop and see if ARD Admin still has startup issues. If it starts working properly you can trash the com.apple.RemoteDesktop folder. Note this will reset ARD and lose any custom UNIX commands you may have set up, back those commands up first before trashing the folder permanently.