I keep forgetting the path to this schema folder all the time ...
C:\Program Files\Microsoft Visual Studio .NET 2003\Common7\Packages\schemas\xml
And this is about what you need to have in the schema. Found the following in the AdRotator example from MS.
<xsd:schema vs:ishtmlschema="false" vs:friendlyname="Ad Rotator Schedule File" xmlns:vs="http://schemas.microsoft.com/Visual-Studio-Intellisense" elementFormDefault="qualified" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="http://schemas.microsoft.com/AspNet/AdRotator-Schedule-File"targetNamespace="http://schemas.microsoft.com/AspNet/AdRotator-Schedule-File“>
[Update: Actually the target namespace and the xmlns attribute for namespace is also not required. The vs: attributes and the xmlns:vs namespace should however be included]