Hi Frederick, an open joined surface is a set of surfaces that are glued together at some edges but not all, there are some unjoined edges (also called "Naked edges").

To get it to be a solid will involve investigating the areas where there are naked edges and possibly doing some repairs in those areas and joining pieces together.

You can highlight naked edges by setting up a script on a shortcut key as described here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2

Or also in V4 if you select the object and push the "Details..." button on the object properties dialog it will give a report of how many joined and naked edges there are and you can click on the label for the naked edges to select them.

There is a tutorial on those kinds of object repair techniques here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

If you can post the model file or e-mail it to me at moi@moi3d.com I can take a look and see about any other more specific advice.

- Michael