'Badly' in this context means 'very much' in the sense of 'urgently'.
eg He badly needs a haircut = His hair looks terrible, and he really needs to get it cut as soon as possible.
Your sentence means 'I desperately want an ice cream'.
2014년 6월 3일
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The word "badly" gives a sense of urgency so the person or people you're talking to will know that you really, really want to eat some ice cream. (.^_^.)/