We are aware that the process to get a Mac version for amending is not updated yet, and have forwarded that concern to the Moderators of the forum.

To get the full Mac version you will need to contact support directly.

As stated, you don't really need to amend, you need to add a state to your existing return.  But you still need the Mac program from customer support.  You may have to pay a fee to download the state module to your computer even though customer support should provide the federal program for free (assuming you already paid once for Turbotax online).