Have you installed Epsilon in the environment where you are deploying your plugin or are you exporting/packaging some of the epsilon plugins with your distribution? In the first case the "org.eclipse.epsilon.commons" should be in your environment and as Dimitris pointed out a reproducible list of steps will help. If the case is the latter, you also have to export/package the "org.eclipse.epsilon.commons" with your distribution, as the dependency is only a requirement and is not used to export/pack additional dependencies when building.
Horacio Hoyos Rodriguez
University Of York