I used Optinmonster at my previous company to capture email signups for our newsletter. What we did was create a smart list in Marketo to capture the addresses to report on each month, and then added them to the email subscription. I did not use the Optinmonster reporting as I used the Marketo smart list and Bizible to report on the number of signups via the Popups and Sidebar widgets we used through Optinmonster. I felt I could get better/deeper data this way. Hope this is helpful!