http://technet.microsoft.com/en-us/libr ... 98177.aspx
Supporting connectivity with public IM users in your organization requires that the appropriate licensing is in place and that the provisioning process for the public IM services that you want to support is complete. Provisioning can take up to thirty days, so allow sufficient time to complete the process prior to the date on which you want to implement support. For details about licensing requirements and how to complete the provisioning process, see the "Public IM Connectivity Provisioning Guide for Microsoft Lync Server, Office Communications Server, and Live Communications Server" at http://go.microsoft.com/fwlink/p/?LinkId=155970.
If you are implementing audio/video (A/V) federation with Windows Live Messenger, there are two parameters that you need to modify: the Microsoft Lync Server 2010 encryption level and the EnablePublicCloudAccess policy. By default, the encryption level is set to Required. You must change this setting to Supported. The EnablePublicCloudAccess policy is set to false, this needs to be set to true You can do this from the Lync Server Management Shell.
Start the Lync Server Management Shell: Click Start, click All Programs, click Microsoft Lync Server 2010, and then click Lync Server Management Shell.
From the command prompt, type the following commands:
Set-CsMediaConfiguration -EncryptionLevel SupportEncryption
Set-CsExternalAccessPolicy Global -EnablePublicCloudAccess $true -EnablePublicCloudAudioVideoAccess $true