If you are using two different versions of MindNode please note that MindNode 7 introduced a new file format. This file format is unfortunately not supported by legacy versions like MindNode 2 for Mac and MindNode 4 for iOS. In order to revert back to a file format compatible with the legacy versions please open the document in MindNode 7, export it as MindNode document and enable legacy version support in the export window.
In MindNode 7 for iOS, it is also possible to set the legacy file format as a default. For enabling that please open the MindNode settings, shake your device, select “Show Settings” in the dialogue that appears and then enable the option “Create new documents in legacy format”.