You are missing the a bit of the assembly name:
It should look something like this:: IWImportAssembly, XSLT_External_Assembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=c5b7d5a4522ce583
Note: the namespace used in the previous post was just an example, each assembly in the extensions xml file needs to have it’s own namespace. I would suggest using a better namespace that reflects your environment. The urn style is just a personal preference, the namespace can be any unique string e.g. urn:yourcompany.com:utilities:objektimport or http://yourcompany.com/Utilities/ObjektImport