Recently a client handed me a .mp4 file of a long interview that they received from the interviewer. I let it encode in Compressor to DVD only to find out that it was PAL when I went to import it into DVD Studio Pro.
The quickest way to check if something is PAL is to open it in Quicktime and then hit Command + I to open the HUD Info Window. If you see "FPS: 25" it's likely a PAL video.
In compressor, after you've brought the video in, and set it's settings and destination(s) in the Inspector hit the Encoder button (second from left) and you'll see this:
Hit the little lock/gear button to the right of the grayed out PAL drop down and it should default to NTSC like this:
You should be good to go with your NTSC encode/conversion.