If your calves are at least a couple weeks old just put the feeder near where the cows loaf (near water, hay, etc). Put a couple handfuls of your preferred show feed in the trough. Now for the hard part: don't watch and wait - it will drive you nuts. They will eventually wander into the creep feeder, and when they do they will find the grain. If it gets old and dry just replace it, but do not add much to start. You might also want to throw a bit of good hay in it too.
Patience is the only trick I know, but once they get going they will really go at it!