Cause: This is a known issue with Lync for Mac 2011 14.0.0 release.
Solution: Install Microsoft Lync for Mac 2011 14.0.9 Update to sign into Lync with your work account.
The most current version of Lync for Mac 2011 is also available from Microsoft AutoUpdate. AutoUpdate is a program that automatically keeps Microsoft software up-to-date. To use AutoUpdate, start Microsoft Lync and make sure it's selected. On the Apple toolbar, click Help and then click Check for Updates.