You'll have to play the song "Satellite Radio" on at least Hard difficulty for this achievement, as Easy and Medium don't have any hammer-on notes at all. (Hard is recommended, of course, being much easier than Expert.) You can unlock this by playing either bass or guitar; bass has fewer hammer-ons, which may make things easier on you.
You'll also have to actually hit the hammer-on notes by hammering them on rather than strumming them. I went ahead and strummed my way through the whole song worth, got 100% of notes hit... and had nothing to show for it. Not sure if the pull-off notes also can't be strummed; for now, I'd recommend the better-safe-than-sorry approach of not strumming them as well.