Message to User:
You're missing quite a bit of capitalization.
#showtooltip Lightning Bolt
/cast [modifier:alt] Elemental Mastery
/cast Lightning Bolt
as far as I know, you wouldn't need the #showtooltip line in this macro.
normally you only need it, when you want something different shown on your button, then what it actually does.
So in this case, deleting the first line, the button would show the LB icon, and when pressing the ALT key, the EM icon would appear.
but as a tipp, since the last patch, you can add conditionals to the #show, and #showtooltip command.
to stay at the above exemple, you could try this:
#showtooltip [modifier:alt] Elemental Mastery; Lighning Bolt