You can't. Not exactly, anyway. Attached is you're drawing, with the solid, and a few steps in between.
If you're using MeshCAM, there are 2 ways to get you're model into MeshCAM. The easiest way, is to use the 3DFace command and save as a .dxf. Change to a different layer, and, using the 3dface command, pick the 3 or 4 corners of every face in a clockwise direction. After you've done all the sides, delete all the lines on the original layer and save as v12 .dxf. This works with 3 or 4 sided faces only. You can use the mesh commands to do similar with curved surfaces. Just remember to explode meshes before saving.
Way #2 (Solid). There are 2 basic ways to start to get a solid part. You either start with the solid primitives, or you extrude or revolve lines and shapes into solids. After that, solid modelling consists of subtracting and adding multiple solids to and from each other, as well as slicing, chamfering and filleting.
Step1) I took the outline of the base of your part, selected all the lines, and used the REGION command to make it a region.
Step2) Extrude the region to the height of your finished part. (To extrude something, it has to be either a region, or a closed object - see the help file for more info.)
Step3) Use the Slice comand to cut away what you don't want.
When exporting to .stl file, the model must be in the positive octant of the UCS. That's where it sits in the drawing.
It's a good idea to look through the help file. The user guide has some good info on creating 3D objects. If you're doing solid modeling, you only really need to know a few commands:
These will let you do probably 99% of what you can do with AutoCAD solid modelling.
Here are some good tutorials. There listed newest first. I's start at the "Modeling Primitive 3D Solids",August 14, 2003, and work your way up from there.