http://scn.sap.com/community/scm/transportation-management/blog/2013/03/21/tmp-the-tm-podcast
Currently you are in the root node. The fields are in the node instance editor.
If you click the ITEM in the left pane, you can get the fields of ITEM node.
Also in some cases, these may not be straight forward and you may have to create data crawlers to fetch value from certain fields.
It wont be possible to explain the entire concept of various business objects of TM, in a thread like this. Above link may be helpful.
Note:
The TCode is BOBF/TEST_UI [My mistake]
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