I spent already too much time in this. So I hope I can save you some lost time!
First, you can not use it in a normal Office 365 Preview account (prof, ent. etc.) The only place you can add apps under Organisation(other places you can only see the apps and click refresh) is the Office 365 Preview for Developers. With this preview you can have only one user!
I opened a Office 365 Preview for Developers account through MSDN, but even if I install the app on Organisation level, then activate it in OWA, I don't see anything Harmon'ie in OWA. Neither in email with attachments (PDF, WORD, etc) or new emails where I want to insert document(links) from my team site.