MacBook Pro 13" wont connect to wifi
At the lower left corner of the Network page in System preference, there's a "+" button. 
If you click it, you will have a drop-down window asking you how you'd like to create a new network service.
From there you should be able to select "Wi-Fi" as its interface, and give that service a name.
Then click "create", and you should have the "Wi-Fi" tab back.
Then in the Wi-Fi tab, at the bottom there's checkbox "Show Wi-Fi status in menu bar", check it and the Wi-Fi symbol will be in the menubar.


After all these steps you should be able to connect to Wi-Fi networks just as you usually do.