If, like me you have installed apps using "snapd" you will find that they cannot be added to cairo-dock whilst running .... nor can they be added from the launch-menu because they are not there either.
The solution is to make a .desktop entry in /usr/share/applications.
The following example is for "gimp" but can be copied and changed to any other snap application.
create the enntry with your favorite editor (mine is nano for simple jobs like this):
"sudo nano /usr/share/applications/gimp.desktop"
and add the following text:
GenericName= Gimp for Linux
After which (when saved) cairo-dock will prompt you if you want to open the application.