Ollie,
If you are not seeing the Alt (MSL) or Alt (AGL) on the instrument list, then they are already added. There was an Altimeter Setting, which is Baro instrument added in the Beta, but those were a bit premature as we are still working on them.
To see your Altitude in a digital format, the instrument you are looking for is the Alt (MSL). Keep in mind this is your GPS Altitude and your aircraft altimeter is pressure altitude. They will be slightly off, but offer a good reference. We are working on a Baro/Pressure Altitude solution for a future release. Stay tuned.