Hi Involute, the main way to find out where your object is open is to set up a shortcut key which will select naked edges, see here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2

If you do that on your model you should see 2 edges highlight down here:


There isn't any "side wall" for the hole there you'll need to build one. A quick way to do that is with the 2 edges selected run Construct > Loft and uncheck the "Cap ends" option before you finish the Loft command. That will build a cylinder surface between the edges, select that and your main object and use Edit > Join and you should then have a closed solid.

- Michael