How do I change or activate the auto signature?

If my friend emails me from an IPhone it says sent from my iphone, or if someone emails me from their BlackBerry it says so....mine doesn't. I found in the email settings the auto signature but not how to turn it on. I want it to say sent from my blackberry!

http://forums.crackberry.com/f3/how-...utotext-38096/

This will help with emails: http://forums.crackberry.com/f3/how-...ackberry-4980/