Plug the power adapter into your Mac, then press the power button to power it on.
If you do not see “Active Mac” and the system boots directly to the Welcome screen, please proceed to the next stage.
At the Recovery Assistant Screen, click on the Wifi icon in the top right to connect to your wireless network. Click Next
Enter your Wifi password, then click Join.
Wait for the "Your Mac is Activated" Window. Click Restart
Wait for it to reboot to the "Welcome or Hello" screen.