DealCredits are used to send mail!
They're also used when you press Enhanced Search (to get phone numbers, emails, and additional addresses for the owner).
Just go to More > Settings > Account > Set DealCredit Reload Amount
Click on the drop down, select an amount, and click the Check Mark or the Update Default Reload Credit Amount Button.
Note: The IOS version of the app does not have the Account option. Please go to DealMachine.com from your computer or your mobile web browser to access it.