Will a Microsoft Office Add-In developed in Visual Studio work for both a Windows and Mac ( osx ) versions of Ms Office?
Office:Mac 2011 only supports add-ins that were developed in VBA.  Office:Mac 2008 does not support add-ins at all.  A new version of Office:Mac is slated for release in 2015, there have been no announcements as of this writing (December 2014) whether Microsoft will support additional methods of developing add-ins in the new version of Office:Mac.

For further information, check out Getting Started with VBA in Office for Mac 2011 from the Office:Mac 2011 help files.  You might find more assistance in developing Office:Mac add-ins in the Office:Mac help forums hosted by Microsoft.