Just in general, you're catching a lot of sarcasm because it's obvious you don't have a clue. The things you're asking to do can't be done with a magic wand or an easy button, you've got to put in your time to learn the software.
The seat cushions can be as easy or as difficult as you want to make them. Generally, complex shapes are created with lofts and fill surfaces, and a combination of surfaces and solids. If you want a better answer, you'll have to provide better information. You might be able to fake it with extrudes and fillets, but it's a definite and noticeable fake. Depends on what quality of result you need. Do you need manufacturing data or just a prop in a rendering?
STL files can be imported as graphics bodies, solids, or surfaces. Once you try the solids and surfaces options, you'll know why this is a miserable format to deal with. You're not able to render it because you've only imported as a graphics body. To reimport as a solid or surface, when you're in the Open dialog with the type switched to STL, hit the Options button and set it to surfaces. I know you'll want solids, but depending on the size / detail / quality of your model, it might take you most of the weekend to import as a solid.