You have to use the messages stored in the '~/Library/Mail' folder. Select the right folder, and use the 'Messages' folder right inside.
I have 20 email inboxes that I monitor. I have not been able to get the dredge to work at all. I have tried /mailboxname/Messages, the mailbox name alone and several other combinations but I just can't get it to work.
Can you let me know how to get the dredge to work in my environment?