Can I get this DLC or my money back?
That's odd. Have you tried logging in and out of your hero academy account?
As Zeke mentioned, try logging out of your HA account, and restart Steam entirely (make sure it's not still running in the Task Manager). Then reopen Steam and HA and log in again. If that doesn't fix it, I'd contact Steam Support to see if there's a problem somewhere with the content you purchased no being unlocked or downloaded somehow. This isn't something we've seen from our end.