property Surface.OLEDropMode as exOLEDropModeEnum
Returns or sets how a target component handles drop operations

TypeDescription
exOLEDropModeEnum An exOLEDropModeEnum expression that indicates the OLE Drag and Drop mode. 0 means no drag and drop support, 1 means manual support.
In the /NET Assembly, you have to use the AllowDrop property as explained here:

By default, the OLEDropMode property is exOLEDropNone. Curently, the control supports only manual OLE Drag and Drop operation. 

See the OLEStartDrag and OLEDragDrop events for more details about implementing drag and drop operations into the ExSurface control.