MacBook as WiFi extender
According to Dori's answer in this Apple.SE thread, what you're looking for is not possible:


  In order for a Mac to create a local Wi-Fi network, it needs to have a
  wired Internet source.


More information can be found in this Apple Support KB:


  
  Choose Apple menu > System Preferences and click Sharing.
  Select Internet Sharing and click the checkbox if it isn’t checked.
  Choose the Internet connection you want to share from the “Share your connection from” pop-up menu. For example, if you’re connected to
  the Internet over Ethernet, choose Ethernet.
  Select how you want to share your Internet connection in the “To computers using” list. For example, if you want to share your Internet
  connection over AirPort, select AirPort. If you share your Internet
  connection using AirPort, click AirPort Options and give your network
  a name and password.
  


You'd need to hook your MacBook up to a wired connection before you can share Wi-Fi via Internet Sharing.