Notice: The onboard WiFi Adapter works only with 2.4GHz (b/g/n) wireless networks and does not work with 5GHz wireless networks!
From the your home screen go to System>OpenELEC or Programs>OpenElec Configuration.
Under Network Tab be sure that Wireless Networks are active and Wired Networks are inactive
Under Connections Tab select your network.
When the pop up menu appears hit Connect
When the Virtual keyboard appears type in your WiFi network password and then hit Done.
If your password is accepted you should see issued IP Address so that means you are connected to your WiFi network.
Things to check if wireless does not work:
– Check there are no spaces and strange characters in the network name or passphrase
– Check your network is not hidden
Here is a Youtube video synopsis of all of the above