How exactly can I put a cameo into a .mix file?

I followed the instructions in the "Adding icons guide" of XCC webpage shown as the following:

Start XCC MIX Editor.
Open cameo.mix.

Choose open....
Load it from your RA2 directory.
Insert the icon.
Start Windows Explorer.
Drag-and-drop your icon to XCC MIX Editor.
Choose compact.
Choose close.
Close XCC MIX Editor.

I found the button of "compact" was grey. I could not press on it.

Can someone show me all steps in detail?