Thank you for your message.
Am I right in assuming that you're trying to connect to a remote Host using the Custom Server Security authentication method? If this is the case, then, unfortunately, address book sync and authentication (Custom Server Security) via RU Server are not supported in Viewer for Mac/Linux yet. Right now it's only possible to use Single password and RU Security authentication methods when connecting to a remote Host from Viewer for Mac/Linux. However, we will definitely implement these features in our subsequent beta version updates. Sorry for the inconvenience.
For more information on features included in the most recent Beta version please see this post
Hope that helps.