Control-click the person in search results or download or upload windows & choose Chat. If the option to chat with them is not selectable it means the other person does not have chat option enabled. As the notes I gave suggest, it's best to choose a column which shows those with chat enabled. If they do have it enabled then a little smiley face will show.
BTW I did not say Command-click, I said
Control-click. That's the key2 keys to the left of Command key & is next to the Option key. It has Control written on top of the key.
http://www.gnutellaforums.com/110142-post2.html