As far as i know libgdx, you cannot get the vertices you need to create a Box2d-Shape directly from a MapObject. You can only get the Properities from the specific object.
So you can either store the vertices in your MapObjects properties, or you hardcode them.
To create a ground in your game you can use the ChainShape as specified here: Libgdx JavaDoc
A nice Tutorial on how to create a ground with ChainShape:http://www.youtube.com/watch?v=BEyu55aDhkY
Another possibility is to create a meta-data layer an color the tiles, you want to collide with. Then you can get the TiledMapTileLayer and from that get the Cell that is not null. This only works for box shaped grounds so maybe ChainShape fits you better in this case