Please follow these instructions to install your SSL certificate on Microsoft Forefront/TMG:
Install your SSL Certificate
1. Once you have installed your SSL certificate on IIS, export it under PFX format (p12 or PKCS12).
2. On Forefront, choose: “Firewall Policy”
3. Go to the “Toolbox” tab (located on the right of your screen) and click on the "+" of the “Web Listener” folder. Then, right-click on the Web Listener you want to install the certificate on and select Properties.
4. Go to the “Certificates” tab and click on: “Select Certificates”
5. Click on the certificate you want to install. Then, check that there is no problems with your certificate thanks to the window located under “Certificate Installation Details” and click on “Select”.
Your SSL certificate is now installed.