Hi Alexandra,
Welcome to the board! I'm an InfoPath newbie myself and it would be nice to share and learn from others. So allow me to give this one a shot.
I'm using InfoPath 2007, but I'm sure that controls work much likely the same as that of InfoPath 2003.
What you want to do is double-click on the text box control that you inserted in your footer section (Let's call this "Sum" text box). The Text Box Properties window should then be displayed. On the Data tab, there's a Default Value section and a Value box. This is where you can specify a default value for your controls, in our case, for the Sum text box. Right beside the Value box is the fx button. That's the button described in the tutorial you've been following.
If you still don't see any fx button by following the directions above, let's try Clay's suggestion.
1. We'll do the same actions to get to the Text Box Properties, (double-click on the Sum text box).
2. Under Validation and Rules section on the Data tab of the Properties windows, there's a Rules button, click on that. You want to add a new rule so click on Add.
3. On the new window displayed, go ahead and click on Add Action. In the Action window, select the action from the drop-down menu. In this case, select "Set a field's value."
4. You should have 2 boxes to fill out, Field and Value. The fx button is on the right hand side of the Value box.
There it is. You can now add the function you want after clicking the fx button. Hope this helps, otherwise, please let me know.