Lower nut removal on my Maytag

I am trying to remove the lower arm nut so I can check the filter and food trap. I can’t seem to figure out how to. It does not seem to unscrew. Any help would be great

Hold the spray arm with one hand and try to turn the top portion of the hub (#4 on the [B]diagram[/B]).

Here are the breakdown diagrams and http://www.appliancepartspros.com/partsearch/model.aspx?model_id=5517853

Gene.