creating a summary field from another form in a report

creating a summary field from another form in a report

not sure how to explain this, but here goes:

  1. I have a form called Items. Items contains two fields: item_name and item_price.
  2. I have another form called Master List. Master List contains a number of fields, one of them is a lookup to a third form called Room. 
  3. Master List also contains a lookup field to the Item form, retrieving the item_name.
  4. I wish to create a report on Master List, grouped by Room, that shows the individual rows and gives me a total of item_price.
However,when creating the report, while I can pull in the item_price field, I don't get the icon that allows me to create a summary field, such as Total, Average etc...

How do I fix this?

s.