Configuring Wifi
You can access this from the Retropie menu in emulationstation (you can also access it from option WiFi in the RetroPie setup script):
Then open WiFi
It will open into this menu:
Choose your SSID from a list:
Type your Wifi Password (You may need to wait a bit after you finish for the configurations to save)
After it's done configuring you should see your wifi info in the original menu: