From the MPX4115A datasheet, "Vout = Vs * (.009*P-.095) ± Error", and for reading an ADC, "Nadc = Vadc*255/Vpsu", so given the reading from a READADC (Nadc) where Vadc is from Vout of the sensor, you can re-arrange the equation to get what P is. Vs and Vpsu are known by design ( possibly the same, 5V likely ).
Ultimately you will have something like "P = K * Nadc + C"
Given the equation appears to be linear you could just take two readings, Nadc1 and Nadc2, at known pressures, P1 and P2, and from those determine what values of K and C fit as a first step. I recall that was done using 'simultaneous equations' when I was at school.
From there you can plug P into an equation which gives altitude from that.
It's not clear exactly what you are asking for - A "how to" ( above paragraphs ), how to re-arrange the equations, or how to write the PICAXE Basic commands which will implement those equations.