Whenever I try to create a stair, roof, floor, etc. the item is shown, although the plan is "greyed" out and I can't save the element. I do not have the option to "create" or discard, so how do I create an element that I can save?
Try hitting Save button, this will sometimes "refresh" the Finish button. Or you can go to View>User Interface and uncheck all boxes and re-check, and see if the Finish button comes back.
Make sure you have the latest Update Release installed for your version of Revit, and that your graphics driver is up to date--as these will sometimes solve the problem.
What version of Revit are you using?
Do you have .Net 4.5 installed on your computer? If so, and you are using 2012, have you installed the hot fix that allows contextual ribbon panels/tabs to display properly? If so, and you are using 2013, do you have Update Release 3 installed (fixes same issue)?