use the Exchange Publishing Rule Wizard if you don't already have an Exchange publising rule.
Otherwise, check
Generating SSL Certificates for Exchange 2007 and ISA Server 2006
http://www.isaserver.org/tutorials/Generating-SSL-Certificates-Exchange-2007-ISA-Server-2006.htmlUsing a Commercial Web Site Certificate to Publish Outlook Web Access (OWA) Part 1
http://www.isaserver.org/tutorials/Using-Commercial-Web-Site-Certificate-Publish-Outlook-Web-Access-Part1.html