An MPxNode will never appear in the outliner. The outliner only displays DAG nodes (transforms & shapes). You'd need to use MPxLocator, MPxTransform, MPxField, MPxEmitter, or MPxGeometry to do that.
Your node is always viewable in the scenegraph. The problem is that it's probably not connected to anything that the user is likely to select. For example, if it's connected to an input or output of a joint, you can select the joint and view it's network in the hypergraph. So, connecting into the node network for a transform, shape, material, deformer etc is likely to make it visible. If it's not connected to something physical (i.e. something the user can select with the mouse either in the outliner, viewport, or material editors) then the only way to select a node is with mel. Normally though, you'll write some mel commands that create your node and connect it up properly to normally whatever is selected or going to be created.
The other place you tend to see nodes, is in the attribute editor of a given object. Again, selecting a mesh, transform etc that uses your node in it's history, should display that in the AE. If not, you can use an AESomeNodeTypeRelated script to force it to display.