As rather usual for a Macintosh application, you should drag it out of the downloaded DMG archive, into the "Applications" folder of your OS.
Unfortunately, "intel Core 2 Duo" doesn't tell much about the graphic chipset. But if it is an intel Integrated Graphics Processor (GMA 9xx), your chances are low that you would make PlaneShift run successfully. PlaneShift requires gaming graphic chips, the OpenGL support of intel IGPs (except HD graphics of modern Core i#) is in general "substandard".
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