First you have to download Cheat O'Matic or any other memory scanner.
Download link: http://www.freewarereview.info/pub/omatic.zip
Then open Airport Mania and the O'Matic. On o'matic, select "airport mania"
and then put your actual cash's amount, hit search. Buy or Sell something 
ingame, put the new value on o'matic and hit search again.
Do the last step again to change the value. If everything went alright you
should get a succesful message. Put the value you want to have in the box 
and hit SET. Done!