A key to pptx conversion is related to attempts of conversion between older iWork Keynote presentations (.key) and Microsoft PowerPoint presentation formats (.pptx). This may be a little problematic to achieve as not many programs support Apple iWorks formats.
Nevertheless your best option (except for Keynote itself) is to try the LibreOffice Impress from as it should support at least some versions of older iWorks formats.Open KEY file Open PPTX file
The most popular presentation software for Mac
Powerful multimedia presentation software tool from LibreOffice suite