Until now, signing in on the site has been pretty much optional - in particular, you could set up an alert without logging in, just by entering your email address.
Behind the scenes, we'd silently create your account for you when you set up your first alert or signed up for an Elite membership.
This ended up leaving people very confused (understandably!). Now, you'll need to be logged in to set up an alert.
If you've ever set up an alert or signed up for Elite membership, you already have an account. Otherwise, you should sign up first.