If there is an unspent amount carried over it goes on the Form 2441 in box 14.  Hopefully the software will automatically carry it forward to the subsequent year's box 13.

I have one of these right now and I'm still waiting on documentation from the client's employer showing the amount carried forward.  Not sure how much documentation might be required but with kids involved there's CTC due diligence so I'll err on the side of caution.