Are your two computers on the same network? If so, you can use file sharing to drag the folder between computers.
Based on your question though, it might be easier to use an external drive. Do you know how large the Library is, and do you have a flash drive that is large enough to hold it?
Navigate to the Pictures folder and click on the iPhoto Library, so it is selected, and press CMD-I to see the size. Another option is to right click (CTRL-Click) on the Library and select Get Info.
If your library fits on a flash drive (or external drive you have), you can drag that file off to the flash drive (be careful not to move it out of the Pictures folder on your iMac). It will copy all of the pictures to the external drive.
When the file copy finishes (it may take a while...), eject the external drive by dragging the EXTERNAL DRIVE Icon to the trash, and then plug the drive in the MB Air. When the drive shows up on the Air, drag the iPhoto Library off the drive into the Picture's folder on the Air.
If that isn't an option, we can try file sharing...