Download and install the Microsoft Office Communications Server 2007 R2 Web Service Provider from http://www.microsoft.com/downloads/en/details.aspx?familyid=CA107AB1-63C8-4C6A-816D-17961393D2B8&displaylang=en on your Client Access Server. This MSI package contains the installation programs to the local hard drive. Normally it will put them in C:\Web Service Provider Installer Package, but I've also seen it install to a different drive. Make note of the location it uses during installation. The package will install the following files: Next, download and save the OCS 2007 R2 Web Service Provider Hotfix KB 981256 from http://www.microsoft.com/downloads/en/details.aspx?FamilyID=45C94403-39FA-44D3-BE23-07F25A2D25C7 to the same C:\Web Service Provider Installer Package folder. Download and save the Unified Communications Managed API 2.0 Redist (64 Bit) Hotfix KB 2400399 from http://www.microsoft.com/downloads/en/details.aspx?FamilyID=1F565A42-71D2-4FBD-8AE0-4B179E8F02AB to the same C:\Web Service Provider Installer Package folder. NOTE: If you have problems saying that you need to have framework 3.5 uninstall Framework 4.0 Extended and Client from AddRemove Programs. In SBS 2k11 when you uninstall framework 4.0 This will break  Remote Work Places. To fix Remote work places reinstall .Net Framework 4.0, Go to IIS 7, Expand Application Pools go to SBS Web Work Place. Advance Settings and change .Net Framework Version to 4.0 After reinstalling it.   If your CAS server is running Exchange 2010 SP1 on Windows Server 2008 R2, you need to download and save the UcmaRedist.msp patch in Microsoft Office Communications Server 2007 R2 Hotfix KB 968802 from http://www.microsoft.com/downloads/en/details.aspx?FamilyID=b3b02475-150c-41fa-844a-c10a517040f4. The tricky part here is that the file name (UcmaRedist.msp) is the same as the Communications Managed API 2.0 Redist (64 Bit) Hotfix KB 2400399 you just downloaded. Just rename this file name to something like UcmaRedist-R2.msp. Now install the following files as Adminstrator in this order: vcredit_x64.exe UcmaRedist.msi UcmaRedist.msp UcmaRedist-R2.msp, if your CAS is running on Windows Server 2008 R2 CWAOWASSP.msi CWAOWASSP.msp dotnetfx35setup.exe, if the .NET Framework 3.5 is not installed on Windows Server 2008. For Windows Server 2008 R2, install the .NET Framework 3.5.1 feature from Server Manager. Note that the MSI and MSP packages have a limited GUI during setup and don't indicate that they've installed successfully. Next we need to configure the Exchange 2010 SP1 Client Access Server for Lync Server integration. Run the following two commands from the Exchange Management Shell on the CAS:   $cert = (Get-ExchangeCertificate | Where {$_.Services -ilike "*IIS*}).Thumbprint Get-OWAVirtualDirectory | Set-OWAVirtualDirectory -InstantMessagingType OCS -InstantMessagingEnabled:$true -InstantMessagingCertificateThumbprint $cert -InstantMessagingServerName pool.domain.com Be sure to change the InstantMessagingServerName value in the command above to the FQDN of your Lync Server pool. Now we need to configure the Lync 2010 RC server. Use the Lync Server Topology Builder to add a new Trusted Application Pool, as follows: Open the existing topology. Expand your Lync Server 2010 (RC) > your sitename. Right-click Trusted application servers and select New Trusted Application Pool. Enter your CAS server or CAS array's FQDN in the Pool FQDN field, select Single Computer Pool and click Next. Select the Front End Pool for the Trusted Application Pool. Click Finish. Right-click the new Trusted Application Server and select Edit Properties. Clear the checkbox for Enable replication of configuration data to this pool and click OK. Publish the new topology. The final step is to create a new CsTrustedApplication using the Lync Server Management Shell on the Lync 2010 RC server. Run the following command from the management shell:   New-CsTrustedApplication -ApplicationID ExchangeOutlookWebApp -TrustedApplicationPoolFqdn cas.domain.com -Port 9999 Enable-CsTopology Be sure to change the TrustedApplicationPoolFqdn value in the command above to the FQDN of your CAS server or CAS array. The Port value can be any unused TCP port.Now login to Outlook Web App and enjoy the new Lync Server goodness! Taken from http://www.expta.com/2010/09/how-to-integrate-lync-server-2010-with.html