Today our extension works as a developer only demo, and no Yahoo keyserver service is online yet. If our toy keyserver code didn't make it into this release, you will see it very soon and will be able to experiment.
We released the source so we can solicit security feedback early, and can keep all security and privacy critical parts of this product in the open. No "just trust us" now or in the future.
Along those lines, we will work with industry peers to build public key registry software that interoperates with other federated providers, is open source, and is transparent in the cryptographic sense. I.e. Yahoo and Google users should be able to send encrypted email to one another with ease, and if either company was forced to lie about the public keys of our respective users we would be caught because math.
There's a lot more in the works, and we have many challenges to overcome to make this generally usable. In addition to the key server, there's not unnecessarily breaking existing mail features, multi-device issues, search over encrypted content w/o providers being able to read it, and mobile support to start.
If you know anyone with the skills and motivation to make strong, usable encryption available at internet scale we'd love to listen, collaborate, or hire. :)