Hi there,
If you mean popup as in a dialog bow, then yes its possible but only in the client. If you are using browser forms, popups aren't supported. For browser forms, I would recommend using a section control that contains all of your help information. Then you can use the button control to show/hide this section when its pressed. This will help give the user contextual help next to the section they need to fill out. It's pretty easy to implement with rules in InfoPath. Hope this helps!