One of the easiest way to know where a program is located is by running "which" command with the name of the program you want to locate.
In your case, the command to run is "which aircrack-ng". Not sure if the "-ng" part is necessary.
If the above fail, try this command "which aircrack"
what does the above two commands give you? (second one if the first one fail)
User programs usually go to "/usr/bin". You should start looking for programs there