When you get a text, it should say "Open with..." and you can set a default. If it's already set for the stock messenger to be default, you have to go through a few menus.
Push menu button > Settings > Applications > Manage Applications > All tab > Scroll to Messanger and open > Clear defaults. Then it should ask you which app to make default on your next text.
Also, remember to go into the stock sms app and turn off notifications, so you don't get two reports of the same message.