After a fresh install of OS X Yosemite I needed to install the Lync Web Plug-in for MAC. After downloading LWAPluginInstaller.pkg the install came up with the following error:
"You cannot install Microsoft Lync Web App Plug-in in this location. The Microsoft Lync Web App Plug-in installer does not allow its software to be installed here."
Click on the "Install for me only" selection again and the error goes away.