Hello,
Sorry to hear you are having issues. If you applied an icon package. And, after it is loaded, go to Customize, right click on an icon you want to work on, you should see something like screenshot below. Click on Edit using IconDeveloper that will open up IconDeveloper with that icon in it.

Thank you.
Basj,
Stardock Community Assistant.

As mentioned above you will need to applied an iconpackage first, before you can edit the icons. If you not applied any package it will not work. Looking from your screenshot above you have not applied any icon package yet. That is why The icons show there is still Microsoft icons. Look back on my screenshot. I have my own icon package applied. That is why all the icons on my screenshot is from my EcoFun Pack.

Thank you,

Basj,
Stardock Community Assistant