If you download a package and it has extension a2, a3, a4 or a5, move the file to ModifiedFiles directory in Mod Installer:

The ModifiedFiles folder name depends on for what FC game the mod is created, so for example:

Some mods like Winter Hope County or Libertad mod has own modpack bin file.

To add it to Mod Installer, simply put the file into the ModifiedFiles folder. The folder name can be for example ModifiedFilesFC5 or ModifiedFilesFCND - it always depends for what FC game it belongs. Just like this:

Drag drop adding

Or you can add a package or modpack directly via Mod Installer using drag drop.

Simply drag the package or modpack file (must be unzipped) and drop it to Mod Installer window.