You don't need to use the horn but they should show up after you fight Deathmold with Saskia. Zoltan tells you about their arrival and you can see them fighting by the main gate. As for the letter I never found a use for it either. Think I said I'd tell Iorveth something for her too but don't think anything ever came of it.

You don't actually use these. If you follow the conversation during the battle, the Trolls showed up to help automatically (if you helped them). Though I looked for them, I couldn't see where they were.