I need to amend my Ohio State Tax return to include the Ohio SD100 return. It only shows me a comment about the Federal form to amend - how do I access the state part?

You do NOT amend your Ohio return. The school district return is a total separate filing from the state return and can be filed separately, although it may have to be mailed, instead of e-filed.

TurboTax (TT) does not automatically prepare a School District return, you must initiate it.

At the screen, in the Ohio program, titled  "A few things before we wrap up your state taxes"
      scroll down to School District taxes. 
Or later at the screen  "Your 2018 Ohio Taxes are ready for us to check"
    -Click Other situations
    -Scroll down to School District taxes.

How do I add the SD form if I have already submitted the State Return?  I am past the point of being able to check the box.  I didn't realize I hadn't done it until it was too late?  Any suggestions?

I don't have online software, but it is my understanding, you can re-enter the program and navigate to those screens. Go thru the SD interview to complete the form.
Or you can try filing at the OH dept. of taxation web site:
<a rel="nofollow" target="_blank" href="https://www.tax.ohio.gov/online_services/ohio_school_district.aspx">https://www.tax.ohio.gov/online_services/ohio_school_district.aspx</a>
Or do the form by hand:
<a rel="nofollow" target="_blank" href="https://www.tax.ohio.gov/portals/0/forms/ohio_individual/individual/2018/SDIT_SD100_Bundle.pdf">https://www.tax.ohio.gov/portals/0/forms/ohio_individual/individual/2018/SDIT_SD100_Bundle.pdf</a>