virtualbox.org

I'm on Linux Mint 17.3 and download the deb file for Ubuntu 16.04. I chose to install VB by right clicking on the deb file and going to Properties > Permissions tab, and ticking the box for Allow executing file as program. I then opened the file and sooner than later got this error:

ticking the box for Allow executing file as program

deb files do not need to have the x-bit enabled. They are to be installed with a package manager like gdebi or dpkg.
As for your error message you need to install the package mentioned.