How do I go back to my completed federal form to add my state that turbo tax said I didn't enter?

What do you mean by find my federal? Have you filed already? If so, you will need to amend your return and add the state. 

To amend your return, wait to see if your return has been accepted or rejected by the IRS or state. If rejected, you can correct and re-send your return. If accepted you should wait until your return has been processed and you receive your refund or confirmation that any tax due has been paid. If you file an amended return while your first return is being processed it can cause extended delays for both returns if two returns are in the system at the same time. In addition, if the IRS makes any change on your original return, you might end up having to amend the amendment. Amended returns can only be mailed - allow 8-12 weeks - can take up to 16 weeks (4 months) for processing. This Turbo Tax FAQ will show you how to amend your return.

How to amend (change or correct) a return you already filed. How to amend