When I try to add myself to a film (The Snowman Live 2010 DVD) I click 'EDIT' at the bottom of the page and it just takes me to a login page but I am already logged in. I can't go any further.
You need to either authenticate your account (http://www.imdb.com/account-imdb/ext_...) or verify your e-mail address (https://secure.imdb.com/register-imdb...) in order to be able to edit IMDb data. If you account is not authenticated or verified, trying to edit data will only ask you to log in, as you're experiencing.