I installed Eclipse (both classic and java versions) and added the Activiti plugin via the update site (and locally as well). The new project dialog doesn't show the Activiti options. It looks as if nothing was installed, but the Eclipse installation details show that the designer plugin is installed properly.
I found the solution. I was using JDK 1.5. After instaling the 1.6 JRE it worked. In the user guide JVM 5+ ist listed as required Java version. Maybe the user guide should be updated at least for the BPMN designer.