While not difficult, adding new DDL types into Enterprise Architect is not quite straightforward. I needed SQL 2008 types added separate to existing SQL 2000 and (later imported) 2005.
The Data Modeling page (link) on Sparx site states that the DDL types can be downloaded from the Resources page. Head there and, under Data Modeling Data Types, download the XML file for SQL 2008 (or click directly here).
Once you download the XML file, go to Tools -> Import Reference Data in the Enterprise Architect. Select the downloaded file, then select the name in the list that appears later and click Import. That should be all there is.
Now, when adding a new table in the Data Model, SQL 2008 is available as a DB type, together with all the DDL types.