How do I access the part of the state of California tax return that deals with use tax? That is, sales tax I may owe but was not charged on purchases?

When you return to the California interview in TurboTax, be prepared to go through a large number of screens. Eventually, you will come to one with the title "Untaxed Out-of-State Purchases".

Answer YES and you can enter your use tax on the next screen or two.