Obtaining O-Scope signals in a AC/DC supply - grounding issues
First, read How should I probe the primary side of a SMPS?

To see the PWM signal generated by the IC:


ground probe to ACGND (hot ground)
signal probe at pin 3 of IC


That's the only thing to do to know if your circuit is working (besides checking MOSFET).

However, to see the transformer's output waveform connect probes to the output of the transformer, before the rectifier diode.