Not all dictionaries make it clear, but as others have said, the usual AmE form is leveled.
One dictionary that does express this clearly is Longman (if you scroll down the page until near the bottom, where it deals with level as a verb):
verb (levelled, levelling British English, leveled, leveling American English)
There are many other verbs in -l that follow the same pattern: travel, cancel, counsel, dial, model, signal, etc. In most (possibly all) cases the double-l forms are considered acceptable (if less common) alternatives in AmE. On the other hand, the double-l forms are generally considered mandatory in BrE.