Hi Amber,
To use Lync for Mac 2011 with Office 365, you need to install the Microsoft Lync for Mac 2011 Update from here: http://office.microsoft.com/en-us/office365-suite-help/redir/XT104018595.aspx?CTT=5&origin=HA102815865.
If the issue persists after installing the update, I suggest performing the introductions in the following article and video to troubleshoot the issue.
Article: Troubleshoot Lync for Mac issues in Lync Online for Office 365
Video: Troubleshoot Lync for Mac issues in Lync Online for Office 365
Thanks,
Maggie Li