However we are not at Apple. There is a a built-in way to enable the installation of .deb files on your Nokia N9. It's as simple as this:
Go to Settings application - Applications - Installations - Allow Installations from non-store sources
You can now click on .deb files on your N9 whilst browsing the internet and download them to your phone. However if you prefer, you can download applications to your computer and transfer them to your N9.