3 eggs on one side of the scale, 3 on the other, 3 left out.
If the scales are uneven you can determin which group of 3 eggs the odd one out is in, if the scales are even then it's in the 3 you left out.
now you've narrowed it down to 3 you can put 1 egg on either side of the scales and leave 1 out. If the scales are even it's the one you left out, if not you can tell by the scales!
I haven't really thought this through and have just had a stab in the dark, but i think this'll only work if you know if the odd one out is heavier or lighter than the others!