I forgot to add a medical expense of $420 and cannot find the field to go back and add it; where can I find this?

Have you filed yet?

TurboTax Free Edition does not support Schedule A Itemized Deductions such as medical expenses.  If you want to itemize your expenses, you would need to upgrade to TurboTax Deluxe.  However, keep in mind that deductible medical expenses are those that exceed 7.5% of your adjusted gross income. It is not likely that the $420 would exceed that threshold.

But it might for state.