Hi,
I know this is an old post but thought I would include a solution that worked for me for this same problem.
I tried MPEG Streamclip (the beta version 1.9.3b8 in order to use the mpeg2 connector).
I have a Mac Book Pro retina; Mac OS X 10.8 and tried to convert a .m2ts file to a Pro Res 422 in order to edit in FCP X and it gave me this error:
Error can’t find the first video frame
I then tried Handbrake (a free open source converter) and it worked.