Thanks Henry. Fortunately, I was able to fix the model in Blender, and now I can convert it to a solid by simply selecting it and under Properties > 3d changing the radio button from TC surface to Solid. There were some intersecting faces that seemed to trip up TurboCAD.
So, the process for modifying STLs in TurboCAD seems to be:
1) Fix STL (I used Meshmixer and Blender)
2) Open STL in TurboCAD
3) Select mesh
4) Right-click and select Properties in the pop-up menu.
5) Left-click 3d
6) Change radio button from TC surface to Solid.
7) Make modifications. I was able to slice the model and subtract simple extruded cylinders from the resulting ACIS solid.
I hope this helps somebody in the future. This worked in both V20 and TC2017, so I am not sure I have any reason to upgrade to TC2017 unless somebody knows of any mesh tools I was unable to find in the new version.
I do have one more question, though. When I extrude a cylinder from a circle, it now shows up as a TC surface and I have to change it to a solid to subtract it from my solid. I imagine that one of the steps above made a change to a default that decides what is created when an extrusion is done. Does anyone know how to change it back, so I can continue working like I am used to?
Have a great day,