State tax filing

That's correct, the credit for other states is not entered during W-2 input.  Rather, when you are working on the particular state that gets the credit, Michigan, you will answer that you paid taxes to other states.  

Navigate to State Taxes > Your State Returns > Michigan. In the Credits and taxes section, choose Other credits, then Other State Tax Credits.  Answer the interview questions to take the credit.

You can read more about living and working in different states here: Multiple States—Figuring What's Owed When You Live and Work in More Than One State