VS 2005 did include a designer for XSD-Files.
Where i can find the version for VS 2008?
I only found a preview version for VS 2008 beta 2.
I would recommend against using the dataset editor for an XML schema, there is a reason Microsoft have stopped supporting the schema designer from VS 2005.
We have had no end of trouble with the schemas that were produced from the VS 2005 schema designer, so much so that we have just recently had to re-design them.
I would either use the basic XML text editor and the XML document outline or I would look at XML Spy. This is what we have just purchased (after using the free trial) to validate and re-do our schemas.