The oscillator are the four amps, bottom lefthand side, each with a .1 MF cap from an input to ground. The output is taken upwards and to the right. So breaking this line and inserting your sine wave here will work. Or you can break the feed between any of the oscillator amps and feed you sine wave in to the input. There are circuits to produce a squarewave and a triangular waveform as well. if your frequency is too far away from the design frequency these circuit may need adjusting.
Frank