If you do not already have an instance of Eclipse running on your workstation, you will need to install one.
- Download the latest version of Eclipse for your platform from http://www.eclipse.org/downloads.
- Follow the installation instructions on linked to on the download page.
- To run Eclipse, follow the advice in the release notes, readme_eclipse.html.
You now have a running eclipse instance in which you can install the Activiti designer plugin.