You need the project to have a Eclipse Dynamic Web Module facet. You can try doing this via the project properties dialog and looking at project facets, then clicking the appropriate check box. This may not be available however, so you may need to do the next thing.
Create a backup of your project and follow the directions at dzone.com. This’ll allow you to modify the project facets via your eclipse .project file.
Keep in mind, once you add those facets, you cannot change them back via Eclipse. Definitely make sure you have a backup before starting.