About a month ago, Microsoft announced the release of Lync for Mac. If you have access to TechNet or MSDN you likely have access to download this tool. Among the benefits of the tool, being able to connect to Office 365. Exciting news, yet for some odd reason it was a battle if you had upgraded to Lion (have not heard of anyone having issues that is running on Snow Leopard). Microsoft then released an update for Lync for Mac.
Now, after the update, it was still no dice for me. Upon further research there were two additional items needed. One was removing an “unknown” certificate that made its way to the Keychain. The other was to manually declare the Internal and External servers for it.
Here is a summary then of the “fixes”
1) Get the update for Lync for Mac, if AutoUpdate has not gotten it, run it manually by going to Help (on any of the Microsoft Office for Mac Apps), from there Check for Updates.
2) Check Keychain Access for an “unknown” certificate, and delete it. To get access to this, you need to go to Applications > Utilities > Keychain Access
3) Modify the Connection Settings for Internal and External Servers to point to: sipdir.online.lync.com:443 (some have reported to not need this, in my case it was needed)
Hope this helps if you are having issues connecting to Office 365 and taking on a really great tool across the Mac and Windows platforms.