Go here:
http://www.theaa.com/route-planner/index.jsp
Set 'From' as Preston.
Set 'To' as M1 6FT for the theatre itself, M1 6NE for the Whitworth Street car park, or M1 5EJ for the Oxford Street car park.
Click 'Via point' and enter 'Mancunian Way'
Click 'Get Route'
If required, use the + button to zoom in on the map (although the written directions may well suffice).
If using one of those car parks (and parking after 5pm) remember to get your ticket validated at the theatre, to qualify for the discounted parking rate. (£7 for 6 hours).
Chris