You need to start over from scratch and create a new project. When you create a project, choose Maven | Maven Project. Then when you get to the list of archetypes, choose one of the webapp archetypes (such as org.apache.maven.archetypes:maven-archetype-webapp). After you create the project, copy all of your sources from the old dynamic web application project over to the appropriate directories in the new Maven webapp project.
This might also be helpful:
http://www.sonatype.com/Support/Books/Developing-with-Eclipse-Maven