Re : the constructor UGen.UGenInput() is unidentified

I don't think you're supposed to call the UGenInput(...) constructor. It is an internal class of UGen that represents the point at which other UGens can be connected upstream (inputs). In the UGens package there is a class called LiveInput that inherits from UGen with the following description:

LiveInput is a way to wrap an input stream with the UGen interface so that you can easily route incoming audio through a UGen graph. You can get an AudioStream that is reading an input from Minim by calling Minim.getInputStream.

Maybe that is what you're looking for.