Hi Pwd, you can find out what spots are open on an object by setting up the "Select naked edges" script on a shortcut key as described here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2

Or also on v4 you can go to the "Details..." object properties dialog and push the label showing "4 naked edges".

If you run on that on your object it will highlight these edges here:


If I zoom in there I can see that there is more than one edge in those areas, with a gap between them:


This can be fixed by deleting the current bottom face, then selecting the remaining Joined surface and running Construct > Planar to cap off the bottom. You should then have a solid after doing those steps.

- Michael