Sine wave pwm signal measurement with digital oscilloscope
There are different ways to trigger your scope.


Use your scope in single-shot operation. Trigger once then work on the captured waveform.
Use the math functionality (page 6-21) with a low-pass filter to filter the PWM signal. With the proper cutoff-frequency you should be able to see the sinewave. You could display the PWM on another channel and overlay the PWM with the filtered signal. You should be able to trigger on the math channel. Update: Triggering on the math channel is not possible with this scope.
Use an external trigger. The scope has a trigger input that you can use. You could program your microcontroller to generate the trigger signal on another pin that you can feed to your scope (see chapter 5).
Filter the PWM using a two-stage RC-filter and trigger on the sinewave. Use two channels the see the PWM and the filtered signal.


Actually you should try all of these since this a nice exercise that will help you to get to know your scope better.