Sorry my last post was completely wrong. You only need to specify the assembly name:
<ExtensionObject Namespace=”http://immowelt.de/Import/ObjektImport” AssemblyName=”XSLT_External_Assembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=c5b7d5a4522ce583″ ClassName=”IWImportAssembly.IWObjektImport”/>
Is your XSLT_External_Assembly in the GAC?
Are you referencing this assembly in your BizTalk project – you should not. If you are remove the reference, add the assmbly to the GAC and restart Visual Studio.