Steve,
I've had the same problem you describe. I could not get a button on a form to press when I hit "Enter" from the field I was filling out. From inside the form, you have to go into the "Browser" tab (by default, it's on the lower left of the Designer window) and make sure that the button you want to have activated when Enter is pressed is higher up on the list than any other button.
However, I've found that the JavaScript already shown in this topic is the most effective way to make sure your button gets pressed.
On a side note, I have a form that will not activate a specified button even when I have used the JScript above... My button and the field that is supposed to activate it are both in a panel. As soon as I added them to the panel, could not longer press "Enter" to activate the button.