Here’s what you actually need to do to install Aircrack for Ubuntu :

1. From your Ubuntu Desktop, go to System > Administration > Synaptic Package Manager
2. Click Search, then type in “aircrack”
3. Click the checkbox next to “aircrack-ng”
4. Click the Apply button
5. Wait for Ubuntu to download and install AirCrack automatically for you
6. Look for the AirCrack icon under Applications > Internet

or just simply type this :

sudo apt-get install aircrack-ng