I suspect that when you are right-clicking, you are doing it where you have some type of Element on your page and you are bringing up the properties box for that element. Click in a blank area or off to the side and you'll bring up the properites box for the Page itself. There you will have a few tabs along the top. Meta Tags is the one you want for inserting keywords and discriptions.
You have to do it BV of course, not via a webview of your page. Like anything else, once you alter it, it will need to be Saved & Published again.
Hope I've interpreted your problem correctly.
Oh, to answer the Question ...... No you don't need to insert keywords or page descriptions before building your site. It can be done at any time.

Rick