IMetadataHelper implementation in ArcCatalog

I want to implement IMetadataHelper interface in ArcCatalog, but i could not find a way to implement it.
API Reference mention that it should be implemented through MetadataExtension but i coud not implement MetadataExtension also.
http://resources.arcgis.com/en/help/...000000w3000000

Is there anybody to help me to solve this issue.

Thanks,

Suleyman.