If you have not yet created a Certificate Signing Request (CSR) and ordered your certificate, see
Certificate CSR Creation Instructions for OCS.
How to install your Certificate on your Windows Office Communications 2007 Server
Open the ZIP file containing your certificate. Copy your_domain_name.cer to the Office Communications Server where you generated the CSR.
On the server, click Start, then Programs, then Administrative Tools, and click Office Communications Server 2007.
Expand the snap-in until you find the Enterprise Edition Server.
Right-click on the Office Communications Server where the CSR was generated previously, and click Certificates.
Click "Next", then choose to "Process the pending request and install the certificate".
Browse to your_domain_name.cer and click Next. This will install the certificate. You can view the certificate and close the certificate wizard.
Your SSL Certificate is now installed and ready to use.
For more information, see Microsoft's official Office Communications Server documentation, located at http://r.office.microsoft.com/r/rlidOCS?clid=1033&p1=library