How can I force Steam to download DLC?

Right click on the game you're after to open properties, click on the "Local Files" tab, click on "Verify Integrity of Game Files" let it do its thing. Once complete if should say it's missing some files (your DLC) and it'll go off and download them.

I had this issue in the past. There is no guaranteed way to trigger the download. Sometimes it will even launch the game without downloading the DLC first. What mostly helps, is to quit Steam completely and then start it up again.

Sometimes also unchecking the "install" check box and checking it again helps.