Hi Naomi,
After reading over your post, I realized that you could be referring to Leads for Customers with 'Is Lead Status Only' checked under Additional Info tab section, in the Customer List. If this is the case then your Master table should be Customers and your Detail Table 1 should be Activity.EntityRef.
You would also need to filter the report when adding your 'Generate Report' action to show only Customers where 'IsLeadStatusOnly' equals yes.
Review webinar 8: http://www.methodintegration.com/cs/blogs/methodblog/pages/Method-Community-Webinars.aspx. It shows you how to pull activities for all customers, so you would only need to add the filter to the 'Generate Report' action to see Customers with 'Is Lead Status Only' checked.