Despite the fact that MS Visio is a part of MS Office suite, the Mac version of MS Office doesn’t contain MS Visio. Latest release of MS Office 2008 for Mac contain only Word, Excel, Powerpoint and Entourage applications.
So, there is no Visio for Mac now.
But don’t lose your heart! There is a way to share your Visio documents on Mac. To work with MS Visio documents on Mac you can use alternative software. You need to save your Visio document as *.vdx file (in Visio XML format) and find any diagramming software which works in Mac OS X and supports import of Visio XML files.
For example, you can use ConceptDraw Professional which is crossplatform (works both in MS Windows and Mac OS X) and supports import from and export to Visio XML. So you have an ability to work with Visio XML files on Mac just as with regular documents.
If you already have a vsd file and you don’t have a Visio installed you can use the free of charge Visio files converter.
Use the standart Open dialog to import your Visio XML file into ConceptDraw then save it as regular conceptDraw document and work with this document in Mac OS X as well as in MS Windows.