The Agile Platform does not supports SQL Server 2008, yet. If it did, the installation checklist would have that option. The support for SQL Server 2008 is scheduled for release with the version 5.0 of the Agile Platform, later this year. To know the Agile Platform requirements, please check the System Requirements technical note available in our Evaluation Center.
Meanwhile, if you are trying to install the platform for your own development, I suggest you install the Agile Platform Community Edition, which does supports SQL Server 2008.