Seems like another plugin you're using (or that i included in myEclipse) uses another version of graphiti, which are incompatible with the version activiti requires (don't know from the top of my head what version this is). I'm not sure if it's possible to force a plugin to ignore a dependency-version or perhaps you can modify the plugin so it depends on the version that you have (this can cause compile/runtime issues off course, but it's worth a shot).
