Not a real answer, because i don't know anything about OSX and key remapping from Mac's, but - do you realize you can define a new struct using the left top symbol button as well?
The catch is, ida lets you drag the button bar anywhere on your screen where it might get hidden; if you do this accidentially, IDA gives you no hint at all about getting it back. So, in this case, there seems to be no way to define structs at all. Windows/Reset Desktop should fix it though.
If you knew about this, sorry about stealing your time, but if you asked the question because you had no way to define a new struct, it might still be helpful -- doesn't give you the convenience of a hotkey, but at least a way to define your structs.