These are the things that I found helpful in gettting alters to communicate:
You, the host, or whomever has to grow to accept the existence of the alter. Work on recognizing and accepting the existence of the alters. (Make friends with them, if possible).
If you don't like the alter or vice-versa, then you'll need to build up a tolerance of each other. I found that eventually, that I (the host) found that they had something to offer me that was better than being at odds with them. They didn't choose to be alters. You have probably existed this way for quite awhile. You can't undo this process over night.
I have also found that by having them come out for longer periods of time in therapy that they learn that it's safe to come out and safer to begin to communicate with you and the therapist. It also seems as though they get used to being a part of the "scene" so to speak.
The more familiar you become with them, and accept them and them, you, the greater the pathways of communication will be.
Hope you find this helpful.
Shrinkrapper