I am trying to add some Wind Definitions but the code is showing up as Custom and it is grayed out preventing me from selecting the appropriate code. The Generate button is also grayed out. Can you please let me know why ?

Due to some recent changes made to the graphic user interface, you will need to select the code from the list of available codes in the first step when you add the Wind TYPE as seen in the picture below. 

If you do not do that, you will not be able to select the code later and the Generate button will also be grayed out. By default the Code is set to Custom in the first dialog as seen above and that is why you are seeing the code set to Custom in your case.