How can I enter private secondary school tuition for my Minnesota state return if I cannot enter it in the federal information because it is not an option?

As you work through the Minnesota return, watch for a page titled "Let's Check for Minnesota Nontaxable Income".  This section also covers certain credits, including the k-12 Education Credits.  Respond 'yes' on that page and continue through the following pages.  You will eventually land on the K-12 Education Credit section.