I am doing a MS Excel software training simulation. I am typing a formula, it would appear that a user has to press Enter after typing text but not after referencing a cell?

Please try to accept that a Captivate simulation is not going to be able to replicate everything that your users might be accustomed to doing in an actual software such as Excel.

Captivate's interactivity is event-based, which means that you can only make something happen when there is an appropriate event that can trigger the action.  There is no event that is triggered simply by the user entering a character via their keyboard.  For certain interactive objects you can set up keystroke combinations that will trigger actions when the user hits those keys.  So you could try having the = key as the keystroke that will trigger evaluation of the Text Entry Box that you are using as a field for your Excel simulation.  But in some cases you cannot use keystrokes because browsers have those keystrokes reserved for their own functions.  These are just typical limitations that come with the territory of elearning I'm afraid.