EclipseLink is not available in Maven central repository, you need to add its repository manually. For example, to use the "full" version of EclipseLink 2.0 (you didn't mention the artifact you're looking for):
<dependencies>
<dependency>
<groupId>org.eclipse.persistence</groupId>
<artifactId>eclipselink</artifactId>
<version>2.0.0</version>
<scope>compile</scope>
...
</dependency>
<dependencies>
...
<repositories>
<repository>
<id>EclipseLink Repo</id>
<url>http://www.eclipse.org/downloads/download.php?r=1&amp;nf=1&amp;file=/rt/eclipselink/maven.repo</url>
</repository>
...
</repositories>
This is documented in the EclipseLink/Maven page.
Regarding the archetype you're using, it's impossible to answer without more details on the kind of project you want to create. And anyway, you can always modify the POM after the facts.