Just a guess but where ever you installed AD2 it's probably not in the
Edit->Preferences->VST.....
You'll see a list/dialog box with at least your Cakewalk vst path. You may to also add you path to the AD2 dll.
Footnote: In the XLN online installer you can change where AD2 is installed. If you change it after you've installed, just run the update and it will move the AD2 install when you directed it to (without doing a whole re-install).