Especially with younger tulip trees (less than 30 years old), flowers tend to be in the top half of the tree. They aren't easily noticed unless you are looking for them.
All Tulip tree flowers possess both female and male parts, as indicated by the link that greene posted.
Seeds do need to be exposes to a cold, moist period (2.5-3 months) before they will germinate under warm conditions (55-80F).
