You must purchase the ArcSDE 9.1 client SDK from an ESRI vendor. There are instructions on how to purchase the client at http://www.esri.com/software/arcgis/arcsde/how-to-buy.html.
Follow the instructions that come with the SDK to install the client to a folder outside of the FDO distribution. Make sure that the folder containing the following DLLs is visible in the %PATH% environment variable .
