Ok, well making polys is easy. As in these screenshots (your UI may look different since everything in Max is user customizable) select the model, then click the create button, then select Face and under Edit Geometry select Create. You will see your model with all the vertices lit up. All you do is click on 3 verts, in order, counterclockwise. You will see the new face. A polygon is nothing more than a collection of faces. Note that all faces have something called a "normal" which basically means the face only "faces" one direction. If you look at it from the other side it is transparent. So, anything you make that has more than one side you can see it from, must have normals facing both ways. You can do this by making another face by clicking the verts clockwise, or by using the normals modifiers. I noticed that your reworked NoHQuv had some faces with only one normal... you could only see them from one direction.
Moving vertices may be better for some situations than moving faces or polys. Select the Vertex button instead of the Face button to see the verts of the selcted object light up.
Note that when you have selected a sub object level (ie: vertex of face) you cannot select another object until you DE-select that sub object.
A quick but dirty way to close an open model is to use the Cap Holes modifier in the Modifiers/Mesh Editing menu. It will automatically close any open sides of the model for you by creating faces. The downside of this is that the Cap Holes modifier isn't very smart, and often does not do the job efficiently or cap off the open areas in the way you'd like. I say it's "dirty" because you always have to go back and fix what it screws up.
One thing you can do to help is run an STL check on the model. STL (STereoLithography) is a process used by companies to take a 3D model and feed it into a computerized laser to create a real object. Lots of automotive and aircraft parts, plastic model kit molds, and other machined items are made this way. Go to the Modifiers/Mesh Editing/STL Check button on the menu. You can set it to highlight edges of missing faces on the model for you, or to highlight multiple faces that occupy the same space ect ect. Many great SFC models out there won't pass an STL check :P although all the stock Taldren ones do.
You can cut the model in half by using a slice plane or a boolean subtraction, and it will make new faces or polys where needed instead of deleting verts that make entire faces dissapear.