Hey there,
A little more information please. What are you using in the "Added" tab? It does not look like DUI / Album List...
In any case, to do what you want with a tree-based library viewer, you would prefer one with a $nodisplay function. This is because to sort the list by descending, you need to subtract the dates to become "negative", but of course you would not really want them displayed that way. So for example -
$nodisplay{$sub(9999,$year(%added%))-$sub(99,$month(%added%))-$sub(99,$day_of_month(%added%))}$date(%added%)
You can use this function with Library Tree (script) for Spider Monkey Panel on either DUI / CUI. Once you have imported the script, you should make a new view pattern for "Added"- right-click the panel > Settings > Panel Properties > see the list of numbered views ("View 07") and add a complete pattern next to it, for example -
View by Added // $nodisplay{$sub(9999,$year(%added%))-$sub(99,$month(%added%))-$sub(99,$day_of_month(%added%))}$date(%added%)|[%album artist% - ]%album%|[[%discnumber%.]%tracknumber%. ][%track artist% - ]%title%
Then switch to it with right-click panel > Settings > 'View by Added'.
(I'm not familiar with the CUI tree components to know if they have a $nodisplay or not? No doubt someone else here will know. It may look very similar to the above!)
(Incidentally, I did see an older post using unicode characters to trick Album List into descending order, but it did not seem to work right, so going with the above advice...)
Cheers