'No AirPrint Printers Found' is a system message generated by the iOS AirPrint feature. It means you are trying to print a document from another applications using the Print button. iOS does not allow Printer Pro to be launched this way.
To send the file to print from another app you need to use the Open In... feature and select Printer Pro from the list of the available apps.
If the application does not support Open In... you can copy the content of the document and print it via the Clipboard tab within Printer Pro.
If the app does not have Open In... and you cannot copy its content to the clipboard (e.g. iBooks) there is a workaround for such a case:
