Well first off you must be a council member of a coven but as you are in Spell Casters you're automatically promoted as such.
So, go to your coven's home page and click on the "View" Net to spell book, then click on "View" next to either the spells, rituals or videos depending on what you want to add. On the next page there should be a button in the top right of the screen that says "ADD SPELL" Click that and it should take you to the appropriate page.
That's how you used to do it back when I was active in 2009 I apologize if something's changed since then but the site seems pretty much the same in its controls so I think it should still work.
Please bare in mind however that if you're not that experienced with magic I wouldn't upload spells in case they're misleading to others. By all means I encourage you to write your own spells but only post spells that you know will be useful to others, otherwise the spell books become cluttered.