The plugin enables Google’s ‘drawing manager’, which takes care of the line/shape editing. It’s not at all intuitive, but I think it’s possible to do what you want:
To delete a vertex: drag an existing vertex over another one and the first vertex is deleted
To insert a vertex: drag any of the existing vertices. This will create two new vertices each time you drag.
As a side-note: if you’re going to be drawing complex shapes, it might be easier to do it in Google Maps or Google Earth and then embed them in the MapPress map as a KML file. Google’s own applications have better line/shape editing tools that are not available in the maps API.