In this case, "badly" is the correct usage; as an adverb, it describes how you miss someone. Just as a note, "badly" here means to an extreme degree, not like the opposite of good.

Got it! Thank you Vincent!

Using "bad" is acceptable in an informal context, eg. I want it so bad, which means I want it so much. Actually, for convience purpose, they tend to use "bad" in everyday conversation. But you have to be careful in formal writing, in which case "badly" is the grammatically correct choice.