CMOS TX to Arduino Nano

So I'm trying to take the 3.3v CMOS TX output of a PerfectFlite PNUT Altimeter (Link to manual) and have an Arduino nano read the altitude levels from it and turn it into numerical value. How would I do this?

manual says connect 3v3, GND and altimetres TX to arduino RX_pin (pin 0 if hardware serial)
Set speed to 9600 baud. Use to read live data from unit

parseInt() will be useful...