It is actually both. Spelled and spelt. I normally use spelled though...

*looks on internet*

Here we go:

v. spelled, or spelt (splt) spell·ing, spells
v. tr.
To name or write in order the letters constituting (a word or part of a word).
To constitute the letters of (a word): These letters spell animal.
To add up to; signify: Their unwise investment could spell financial ruin.