every time i install Unity it installs the plug-in but not the engine i know because when i try to play unity it says failed then i click on install manually then it takes me to http://Unity.com and at the right it says i haven't installed the engine even though I've tried reinstalling it but it still says the engine is not installed can someone help me?

I have a Mac OS X

and every time i try to start unity from applications it closes straight away before anything even opens and still does it when i try to reopen it.