1. Make sure your wall type isn't marked as "Foundation". I had this happen to me the other day. In your options bar you should be able to change from "Depth" to "Height" and then have the wall go from your current level up to what ever height or level you want.
2. It is strange but there is an "Extensions" tab and a "Add-ins" tab. Extensions seem (for me) to only have shown up after I loaded the Revit Subscription Advantage pack. Don't know what else triggers it to show up.
3. I don't know (anyone else?) that you can add loads to elements you don't have in your model. I would suggest either drawing the floors in or using copy/monitor to add them into your model. You'll also want to be sure that the floors are marked as "structural" in their instance properties.