Can anyone instruct me on how to install pd-extended in Ubuntu 10.04 64? I'm having no luck with other sources. Thanks.

I don't think there is a stable release for ubuntu 64bit just yet. But there is a new release candidate for the next version.
http://puredata.hurleur.com/sujet-4340-extended-release-candidate-released

I downloaded and installed the release candidate, but when I run it, it opens Pd vanilla. What am I missing here?

To run Pd-extended in Ubuntu, choose "Pd-extended" from the Programming menu. "Pure Data" will launch Pd-vanilla. As for 10.04/64-bit, there is a build that is an official release, or add the apt repository:

http://puredata.info/docs/faq/debian