Which operating System?
MPLAY has been tested on many types of computers and configurations. You may run MPLAY under any Mac OS version between Mac OS 8.6 - Mac OS X. 
If you are using MPLAY in OS 8/9; make sure you have the latest version of CarbonLib installed (http://www.info.apple.com/support/downloads.html).

QuickTime™ is required since MPLAY uses QuickTime™ for playback. Accordingly, MP3-playback is only possible if QuickTime™ 4 or higher is installed. Therefore, it is recommended to always use the latest version of QuickTime with MPLAY. (http://www.apple.com/quicktime).

68k Macs
There is a unsupported 68k version of MPLAY 1.6 available for download at MPLAYs website. This version will run on old 68k computers like the Performa 630 or similar, as well as Mac OS prior to 8.6. The 68k version of MPLAY is however discontinued. Please note that 68k Macs is not capable of playing MP3's due to hardware limitations!