That is explained in the documentation on Photographer's Toolbox, Jim. The sequence is:
1. Metadata > Save Metadata
2. Run the plugin
3. Metadata > Read Metadata
To explain, the plugin updates the file or its sidecar, but Adobe don't let a plugin update the Exif data in the catalogue. A simple Metadata > Read Metadata would mean that, unless you have auto writing of XML switched on, there's a risk of reading in other metadata like keywords that's older than the metadata in the catalogue, and that's why you need step 1.
I did take to heart the earlier comments about the interface being "daunting" or unforgiving. When I was developing it I was reluctant to make it so simple that it'd encourage people who don't know what they're doing to muck around with Exif data. At the same time, those who do know the risks were telling me they didn't want me to shrink wrap features but wanted a bare bones approach that allowed them to enter whatever they wanted. That said, I may add some extra fields and lenses are a prime candidate.
John