As of now, Kathy's suggestion will work best -- and the MacBU has announced
that they will provide an update soon so that new Mac versions of PowerPoint
can open PowerPoint 2007 PPTX files.
You only save it as PPT or PPS extension.
PowerPoint -> Save As
There is somethings you may like to know when transfering from PC to
MAC.
Graphics and fonts may not be the saem as original design because of
encoding things.