Before creating an IDML of a document be sure that you have accepted all changes first. More information about this can be found here.
An IDML (InDesign Markup Language) is a stripped down version of an INDD. It is often used to make documents compatible with previous versions of InDesign. When using a document with CtrlPrint is important to create an IDML before adding it to the system as this will help avoid any issues caused by old information or plugins associated with the file. This is extremely important if you are using a document created in an older version of InDesign (CS3 or CS4 for example) that has since been converted or if the document has ever used additional plugins other than CtrlPrint.
To create an IDML first choose to "Export" the document by either using the shortcut Cmd+E (Mac) / Ctrl+E (PC) or by selecting the "File" menu and choosing "Export...":
You should now see the following window:
Select to save the document as "InDesign Markup (IDML)" and click "Save".
You can now close the INDD file you have open an open the IDML that you have just created. This document you can now add to CtrlPrint or you can choose to save it as an INDD if you need to do any additional work on the file before adding it to the system.