Well, first of all, do you have a program setup to pass your keys? You'll need something like keyclone to do that before you are even able to set up the macros.

Assuming you have that, the following should probably work:

/cast Regrowth name

Where name is of course the name if your mage. You can get more fancy with using the focus variable and checking nodead and such, but this is the most simple version. Also, reference these links as well for some beginner macros:

The Macro Codex

Macro's for New Multiboxers - A Quick Start Reference