The input is under Expenses > Mission/Program Service Accomp. (990).

When you're lost in the PTO but know what you're looking for, you can simply use the search bar and type in keywords.  In this case, if you type in "mission" (without quotation marks), PTO will list the above as your first choice along with other suggestions.

Another way to find what you need is to click on the Preview forms button, navigate to the field on the form or schedule you want to make an entry for, and click on the blue box (if one exists) - it will then bring you to the input screen.  In this case, if you click on the blue box for Line 1, Part III, of F.990, it'll bring you to the input screen noted above.  However, not all fields have a link and it may sometimes bring you to a screen that may not be totally relevant if it requires multiple entries.  So, YMMV.