you are using a "disc location" view. You can't edit from there as the DVD is read-only.
look in the tree, under "media library > advanced > Cd and DVD". Now, I don't have that viewscheme here anymore, but if my memory serves me right, it uses the [volume name] field. You should find you can perform your edit from there.
On a side note, before you go too much further, here's something else for you to consider...
Importing offline media into your main library can lead to many problems. None of them critical, but most all of them will get very tiresome very quickly.
they will get caught up in your smartlist results for example, and if the the disc is not present in the drive, MC will begin complaining.
MC keeps track of all your offline media in the CD database that's used for the viewscheme I mentioned above. Have a poke around in there, see what you think, and if you have any other questions, just fire away.
-marko.
ps. In case you were wondering, I ditched my Cd & DVD viewscheme in favour of "whereisit?"