You can't...it won't work in CS6 and was only for previous versions running on a PPC Mac (which CS6 won't run on) So...the odds are you have made the mistake of loading older versions of Photoshop plug-ins in CS6, right? You need to remove ALL old Photoshop plug-ins from ANY previous version of Photoshop. It will cause you lots of problems...the ONLY plug-ins you should load in CS6 are those updated for CS6 and installed with the developer's installers.
MMXCore, not AltiVecCore...
Anyway, MMXCore is installed with Photoshop CS6 on both platforms - there is no need to download it.
If you tried to copy plugins from an older version (and are getting a message about MMXCore being incompatible) - go delete the plugins you mistakenly copied from the older version.
Thanks. I did that and also reinstalled photoshop cs6. I then checked the plugins folder and could not find MMXCore plugin.
The plug-ins folder is empty in CS6, and all the required plugins have moved into the required folder.
The plug-ins folder is now just for third party plugins.