Hi Mark,

> I am unable to select the inner surfaces. Whenever
> I try, it just selects the whole solid. What do I do now?

You need to "drill in" to select a face instead of an edge - that's done by doing a second click on the same object.

When you do 1 click on an object, the entire object is selected, but if you then click on it a second time selection will change to a sub-object (a face or an edge).

Edges will take precedence over faces, so in order to drill in to a face you may need to zoom in a ways until the face is a bit larger on the screen so that you have a good spot where you can click on it that is not close to an edge.

Then like Pilou says once you have deleted those inner side wall pieces you can then select the edges on the outside piece which are now no longer attached to anything and also delete those, that's called an "untrim" operation.

Here is a post that describes untrimming in some more detail with an example on a box:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4

Also here is a more complex mini tutorial that talks about using untrimming and re-trimming to repair an object that had a boolean done to it in an incorrect location:
http://www.moi3d.com/forum/index.php?webtag=MOI&msg=446.17


It is also possible to model a covering piece and use a boolean or join to fill something in as well, but it can be easier to use the untrim method when the hole is a kind of isolated thing right in the middle of the base surface.

- Michael