how do i find my Flash drive on my macbook air?

You probably don't have your computer set to show drives on the desktop. Close all your programs and then click on the Finder menu. Select Preferences and then click on the General tab. Make sure there's a checkmark showing next to 'External Drives'. Now your flash drive should appear on the desktop. From there just find your file in your folder and drag it to the flash drive's icon on the desktop.

Do you mean you are not seeing the Drive when you connect it...

If so... Finder > Preferences > General Tab > check External disks

When you plug in the flash drive you will see it's icon on the Desktop. Drag your document from the computer to the flash drive's Desktop icon.

If you cannot see disk icons on the Desktop, then open Finder preferences (select Preferences from the Finder menu of an open Finder window. Click on the General icon in the toolbar. Check all the top four boxes.