I've tried all these suggestions and now i have it working in Kepler; here's how to do it:
I made sure my Eclipse Kepler instance was using Java 7--it was.
The link from the eclipse marketplace did not work. It would show a PMD option for the individual projects properties, but there were no rules to select from. Also the main workspace preferences were not showing up.
I had to install it using this update site, as Jeanne Boyarsky mentioned:
http://sourceforge.net/projects/pmd/files/pmd-eclipse/update-site/
Restart Kepler.
The PMD option now shows up in the Windows->Preferences->PMD
The PMD project menu now shows up when right clicking on the project.