After trying several times, without any success, to create a discussion forum with Tal.ki, I tried Nabble (http://www.nabble.com/) just once and succeeded.
Now I have a forum to embed into the wiki I'm going to use with my students this school year (http://weplayandlearnenglish.pbworks.com/)
Creating a forum with Nabble takes a minute, it is very easy and free. I have created two threads: one for students' introductions and one for asking and giving help with posting in the forum.
Now I can't wait to read my students' first posts