If you have Digital Editions 2.0 (the newer, less reliable version) then use Help/Authorize Computer (or ctrl-shift-U, cmd-shift-U on a Mac)
If you have the older 1.7.2 then use Library/Authorize Computer.
If you have already used it unauthorized, you may need to clean up a little before authorization will work.
ctrl-shift-D (cmd-shift-D on Mac) should do that for you, then restart ADE and authorize as above.