How do I get the HM Dive in "Pokemon Emerald"?

I have already beaten the gym in Mossdeep city, and I have already beaten all the team magma grunts in the space center, but what it wont let me do is battle the team magma members who are talking to steven those members just talk to me! How do i battle the members who are talking to Steven, so that Steven can give me the HM Dive, when I beat those members who are talking to him ?!