There is no standard place in the software. State apportionment is not in any Federal module of ProSeries business (it is in some other professional softwares). So you have to research each state and learn.  

Maryland for example is on Schedule A of Form 510 or 511. Alabama has separately stated items on Schedue B and general apportionment on Schedule C. Louisiana in on Schedule D. Sorry, but there is no simple answer to this. It's one of the hardsest software items I've dealt with. I have business clients in 20-30 states and the tax return is beyond a nightmare due to these apportionments, epescially if the only income comes from a K-1 and you know that state's income, but their law doesn;t work that way.