As @huydnguyen notes above, the SmartThings → Harmony integration is currently broken. We’re awaiting the new cloud-to-cloud integration from SmartThings to fix this. This will allow SmartApps to control the Harmony.
You can, however, get the Harmony–>SmartThings side set up via the Logitech MyHarmony desktop software. This allows you to control SmartThings devices from the Harmony.
See this post for details:
Arghhh - there are too many separate threads on this topic…