When you first run the AlterWare launcher, you will be asked if you wish to download the bonus content. If you answered yes, there is a high probability you do not need to do anything, as you should have already downloaded the various DLC maps when you first opened the launcher.
Since the launcher will only ask you this question once, if you choose not to download the bonus content when you first launched it, you will have to manually amend your mistake.
There are a few options.
Run the launcher with the appropriate command line arguments:
alterware-launcher.exe iw4x --bonus
This will open the launcher and trigger a big download of all the DLCs. Be patient, and the game will open once the download is over.
Pro tip: if you open the launcher with the --help command line argument, you will be shown all the different options.
Alternatively, you need to locate the alterware-launcher.json file in your game folder, open it with your favourite text editor, and change "download_bonus_content": false to "download_bonus_content": true.
After you have edited and saved the JSON file, start the AlterWare launcher as you normally would to trigger the download.