With a mac to take a screenshot, in the upper right corner, there;s a little search glass, click that , and type "grab" , as you type it will give you definitions, just click the highlighted on that says "grab ". Then look to the upper bar there's options, click "capture" you will seeoptions as to what you want to snap a pic of.
or, just apple+shift+4+space , bam scrennshot.
I would get rid of the USB connection alltogether, I learned right off , not to connect this way, supposedly a slower connection, and a couple other things, my kids gonna open there Xmas stuff , gotta go...........