How do I activate or add the automatic voice that declares altitude when landing going forty, thirty, twenty, etc??

Are you using the default Aircraft in FSX/FS9?As i dont beleive this option is available.If you are using PMDG then the altitudes you wish to be announced should be in ADD ONS/PMDG/ when in your cockpit.There may be an add-on that you can get that generates the altitude Annunciations for default FS aircraft but other posters would have a better idea where to find these.

You will likely have better luck posting your question in forum dedicated to the version of FS you are using - i.e. FS2004 or FSX.There are addons available which will provide callouts that can be added to most aircraft.