I assume you're using the external superdrive? Edit: Ooops, I should READ. You should be able to see the disc icon in Finder, and it should have the Eject icon (Horizontal bar + Triangle above it) next to it. Press that and it should work.
Alternatively, drag the disk icon from your desktop to the trash can (it will turn into an eject icon).
I've not used the remote disk facility recently but as far as I remember it just shows up as a local one and can be ejected from the Air the same way as normal.
Alternatively you could turn the Air off so it's no longer shared and eject it from the source machine?