See this link.
Basically, you have to edit the plist for Lync, and enter a registry key equivalent.
The explanation is below:
Basically, the equivalent to the registry keys for windows needs to be entered in the Lync config plist file on the mac, a file named MicrosoftLyncRegistrationDB.xxx.plist that you can find in ~/Library/Preferences/ByHost.
In it, you will need to create an entry for each segment of the reg key path for the DisableServerCheck flag (all lower case). It should look like this:
The value for each of the path segments is "<>" and type string. For the disableservercheck flag, the type is "Number" and the value is 1.
You can use a plist editor (there's a few free ones for the mac, or you can also use Xcode) to edit the plist file. The screenshot below shows you how it should look like.