While I'm on Windows rather than a Mac, are you using the 555 from core parts? If I do that on win it aligns fine. Note sometimes (due to a fritzing bug I think) you have to move the part before it will snap to the grid correctly so you might try that. As well in the view menu make sure "align to grid" is set and grid size is .1 (I think they should both be default but perhaps not, or perhaps they have been changed). If none of that works post the fzz file of your sketch here and one of us will take a look at it and see if we can see the problem.
Peter