The first thing you need to do is install the Web Standard Tools (WST) plug-in. This open-source toolkit facilitates the development of web applications in Eclipse. I warn you that it's not an easy install. It has dependencies on other plug-ins, and it's tricky getting all the release levels to match.
For this reason, if you don't mind upgrading to Eclipse 3.2, it's much easier to install because of the new Callisto project. This coordinates the release levels of multiple projects so that you don't have the installation issues mentioned above. If you want to go this route, go to this site and select the "I build web applications" link.
Yet an easier route is to use MyEclipse, which is a commercial, but very affordable set of plug-ins including a Struts toolkit. The cost is around 30 US dollars per year.
If you decide to use MyEclipse, you can use its wizard to create a Struts project. If you use any of the other options, the best way to start a Struts project is to unpack the struts-blank.war file that comes with the Struts download and import it into eclipse.