If you want to define your own model you can pull one of the available models from Circuit elements/Nonlinear palete. If yo want to use the one form the library you should pull one from Circuit Elements/Libraty/Nonlinear
well I am afraid that you will have to built your own model from scratch using controlled generators etc. if you can't use any supplied from AWR. The problem might be that the model you are going to implement will not be compatible with HB simulator (as this is sometimes the case with user built nonlinear models)