How to install plug ins from Ikea?

Trying to install kitchen design plug in. I downloaded no problem. But, it says to mount the image and follow directions but I don't know what that means. It's called 2020plugininstaller_IKEA.dmg

I looked up in apple support, but when I go to preferences-security-it says to look for a plug ins tab, but there is none. I'm totally confused!

Any help??