You may already know that Office 365 E3 and Small Business Premium includes the full office desktop suite for Windows and Mac for up to 5 devices per user (here), and therefore includes Lync for Windows and Lync for Mac 2011.
You can download Lync for Mac from this page (ensure you are appropriately licenced):
Click “Buy Lync for Mac 2011” for a direct download link (not a link to buy oddly)
Link: http://go.microsoft.com/fwlink/p/?linkid=236302 which prompts you to download from here:
Ensure you also have the latest update: