You have to enable Seam supprot for your project (Properties for the project/Seam Settings)
If you have EAR project and would like to enable Seam support for our EJB/WAR projects then I recomend you to install latest JBoss Tools 3.0.0 (nightly build). It's much easier to enable Seam support for already existing EAR projects there. See Seam Dev Tools Reference Guide.
BTW JBoss Tools Forum is better place for that question ;)
Thanks Alexey, that was just the pointer I needed. Now I have auto-completion working very nicely.
Side comment: if you want autocompletion for entities in .xhtml files, the Hibernate configuration should be loaded (see Eclipse's Hibernate perspective).