Phone or PC Herb? If PC, once the user's name comes up, hit tab to go to the message field and start typing your message. Or, once the user's name comes up, click at the end of his name then click in the message field. I also just clicked directly in the message field and it worked fine for me. What system and what browser are you using?
Could you describe more detail - for example, do you start typing the user's name and the rest of it autocompletes, or do you type the whole name?