Hi Rose,
You can set priorities onto summaries now. However, as the general
recommendation is to never assign resources to summaries, it's unlikely to
be effective! You can change all the task priorities under a summary by
selecting them all and then Project/Task Information and insert the priority
required in the General tab. Remember to select Priority,Standard in the
levelling dialog.
Hope this helps - please let us know how you get on
Mike Glen
MS Project MVP