If you have a full distribution, you already have everything you need.
It’s one of the options you’re offered during Multi-Edit installation, at the dialog where you select which components you’d like installed. Some of the integrations won’t initialize properly if the program they’re supposed to integrate to isn’t there, but you can always fix that up later, after you’ve installed the package. Macromedia’s, in your case. For example, you’re not going to be able to finish the configuration dialogs for, say, Microsoft integration if you don’t have a copy of something like Visual Studio installed, simply because there’s nothing there for the installer to find to integrate with. The Multi-Edit end of the integration will install, though; it just won’t be able to configure until the Microsoft product is present.
However, if you didn’t install one of the AddOns during installation, then the files wouldn’t have been copied into the Multi-Edit installation directory (they all go in subdirectories of <ME_PATH>\AddOns), and likewise, the option won’t show up when you try to install it via Tools | Install Add-On Packages… from the main menu (you’d click on the Browse button, open the AddOns directory, select the Macromedia directory, and click on OK; the option to install the Macromedia integration would then be present in the list).
Whenever you install Multi-Edit, be sure to select any option in the installer you might want at some later date, so that at least, the files will be present so you can install them. If you ever decide you don’t want one of the AddOns, just uninstall it. Then you can delete its files, if you want. However, a complete installation of Multi-Edit 9.10 is less than 40 MBytes, and individual AddOns are pretty trivial in size, so there’s usually not much reason to.