The SBGN plug-in is still Beta software. Nevertheless it's reasonably stable by now, and you can use it for editing and viewing SBGN diagrams. Currently, this is one of the few tools that support all three SBGN languages, and the very latest version of SBGN-ML (Milestone 2). However, the current version of the plugin only works with PathVisio version 2.0.11. We are updating the plugin so that it works in PathVisio version 3.X>X.