Latest versions of Microsoft Visio no longer support VSD format and thus it's very difficult to convert vsdx to vsd unless you have access to some previos version of MS Visio that still supports both of these file types. Or if you own the 2010 version of Microsoft Visio, you will have to download the Microsoft Visio Compatibility Pack from Microsoft Website, which adds support for the new VSDX file format. For this to work you will need to have the 2010 version updated and Microsoft Visio 2013 Viewer might also need to be installed on your system as it shares some functions with the compatibility pack.
Microsoft Visio is not available for Mac, but there are some vsdx & vsd viewers available that may allow users to transform various Visio formats. A good alternative to Visio seems to be ConceptDraw Pro diagramming software which is available for both Windows and macOS users.Open VSDX file Open VSD file
A diagramming program for Apple Mac and Windows
A vector based diagramming software from Microsoft