Hi Bob - a solid is where every edge of every surface is a joined edge between 2 surfaces. When you have any edges that belong only to one surface, that's what is called a "naked edge" and you won't have a solid if you have any naked edges. To identify naked edges you can set up a shortcut key with a script on it as described here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6051.2

When you trigger that shortcut key it will show you all the naked edges in your model , those are the unjoined areas that are preventing things from being a solid. Those spots need to have other surfaces joined together so you've got a joined edge there instead of a naked edge.


You have kind of a long ways to go with this particular model - you've got many surfaces just sticking through each other instead of coming to a common edge, you can't immediately join together something like this which just totally shoots through another piece:




I'm not sure if that's just an extra piece from some previous attempted construction and you just need to delete it, or whether you're trying to incorporate that highlighted surface above into the solid - if you're trying to incorporate it, you've got to Trim all those pieces with each other so that every surface comes to a common edge with another surface, then those surfaces which meet at common edges need to be joined together to make a solid.

Right now you have kind of a whole lot of different surfaces just all sticking through each other and so there is a lot of work you need to do to get them all into a coherent and unified exterior skin before you'll be able to make a solid out of them.

It can be better to try and work with solids at an earlier stage and carve pieces off of a base solid rather than constructing so many different independent surfaces.

- Michael