While playing as Trevor, you will need to complete the two story missions "Mr. Philips" and "Nervous Ron" before you can buy properties around San Andreas. Upon completing the latter mission, the game will inform you about property buying.
You can actually purchase the McKenzie Field Hanger for $150,000 as soon as you finish the "Nervous Ron" mission, as you will have just enough cash and the mission ends right outside the hanger.
Once purchased, you can begin undertaking trafficking missions for Oscar, who you meet after making the purchase. Arms Trafficking missions are performed in the Cuban 800 airplane parked in the hanger and in the BF Dune Buggy parked outside the hanger. Enter either vehicle in order to receive instructions from Oscar. After completing a mission, you will either need to travel some distance and return in order for the next mission to be available, or quick save and reload your saved file. All in all, there are five of each trafficking missions (air and ground) and the achievement will unlock once all ten are completed.