How do I watch a movie I downloaded on itunes on my Macbook air on an airplane without wifi?

If you really downloaded it you can see it without WiFi. You probably looked at it, "in the cloud" without really downloading it, then you have the link but cannot see it without internet connection.

If the movie has a cloud icon on it, you have not downloaded it to your device. You watched it as it streamed. Click or the iCloud icon to download it, then you'll be able to watch it without a cellular data or Wi-Fi connection.

Did you rent the movie or did you purchase it?

If you rented it, you must begin watching it using WiFi so the clock on your rental use can start. Assuming you downloaded the entire movie to your computer, you may then finish watching it without WiFi.