Cannot log on to Lync from Mac (Yosemite)

Hi,

I just downloaded Lync, and whenever I try to log on I get a message saying "Sign in to Microsoft Lync failed because the service is not available or you may not be connected to the Internet". 

I am connected to the Internet, running OS X Yosemite, and Lync has been permitted to bypass the firewall. I have tried to log on using my office 365 username and password. I am not logging on to a corporate account.

Any suggestions as to what I am doing wrong?

Best,

Jostein