Imac update to El Capitan was successful in order to use Malwarebytes but the pkg is not recognizing the OS

I put the Malwarebytes pkg on a thumb drive from my laptop and tried to install Malwarebytes to my old Imac which I just upgraded to El Capitan as required. For some reason its not recognizing the OS which is odd since I used it on my laptop. I tried to install it on my laptop to see what would happened and got the same message. I tried using it on my laptop and got the same message. Luckily I have the version from a few weeks ago but my iMac really needs a cleaning. Thoughts?