Capturing PWM Signal on an Oscilliscope
I've got that 'scope. It sounds like you just need a bit more practice driving it.  To capture a single frame set the 'scope up for "single" in the  Trigger-> sweep menu.. right under "normal" and "auto".  You'll then have to configure the trigger level and all such that you have a chance of triggering on what you want to see.  (Sometimes you'll want to trigger on one signal and look at a different one.. or even two signals.. so you might have to use the external trigger input.)  You may want to look at a lot of single shots to find the one that you want.  Push the Run/Stop button to get another trace. 

As far as getting waveforms into your computer.  I usually just save a bitmap of the on screen picture.  But you can also download a CVS file of the raw data, you've then got to have some other program to plot this data for you.   

I hope that helps,