Don't have aurora in front of me so the labels of buttons may be a little off.
Open up the colony managment screen and make sure Mars is selected, one of the tabs is for terraforming. Select the gas you want to add/remove from the list, then enter the target ammount of that gas in atmosperes. Make sure the check box is ticked correctly for if you want to add or remove gass then hit ok/save.
As you are dealing with Mars the first thing you want to do is to heat it up so either CO2 or 'safe' greenhouse gas is what you will want to add. Off the top of my head I think you want to add around 0.2-0.3 atmospheres. Once colony cost has dropped to 2.0, you will want to stopp adding CO2 and start adding O2 in order to form a breathable atmosphere, as conlony cost will remain at 2.0 while it is unbreathable. 0.1 atmospheres of O2 is the minimum with default race settings. After that finishes you will want to add enough other gasses to reduce the percentage of oxygen in the total atmosphere to 30% or less, probably using more greenhouse gass to continue warming the planet up to the desired temp in the process. If you have reached the desired temp then switch to adding N2 to make up the rest of the atmosphere.
Bare in mind that with only 2 terraformers it will tanke a very long time to finish terraforming, even with 30-40 of them it will still take decades.
