Cause: You haven’t installed the most recent Lync for Mac 2011 update.
Solution: Install the Microsoft Lync for Mac 2011 Update.
Cause: You forgot your password
If you forgot your password, or it doesn’t work anymore, contact your workplace technical support—typically the person who set up your Lync account for you. Provide them with these links:
Cause: Your user ID and password were not typed correctly.
Solution: In the Lync for Mac sign-in window, check your user ID and password for typos. If you’re using Office 365, use the same user ID and password that you use to sign in to Office 365, for example, firstname.lastname@example.org. Passwords are case-sensitive.
Cause: The firewall that is built into Mac OS X is blocking Lync.
Solution: Confirm that your firewall settings are not blocking incoming connections for Lync.
A symptom of this problem is that Lync times out while you are trying to sign in.
On the Apple menu, click System Preferences.
Under Personal, click Security, and then click the Firewall tab.
If the firewall has blocked incoming Lync connections, see your system administrator.