Looks like no one is answering this properly, so here goes.
- Right click on the dmg installer and then choose "Open". You should be able to bypass Apple's checks.
- Launch "Ignition Design Launcher" and add your local designer. Upon opening you will get an "Launch Error Occured, Downloading of launchclient.jar failed" error. Exit from Ignition Design Launcher
- Open a terminal and type "sudo su". Then "chown -R username /Users/username/.ignition" where username is your Mac username
- Relaunch "Ignition Design Launcher"