The asset loaders, located in the Pie Menu, are used to append decals from your Decal libraries to the current scene.
By default Decals will added at the location of the mouse, at the time of opening the pie menu.
This can be changed in the Panel.
Various Decal libraries, represented by Asset Loaders in the DECALmachine pie menu
Using the Asset Loaders
CTRL + D
To quickly bring the same decal into the scene, that you've appended previously, Quick Insert can be used, thereby skipping the Pie Menu completely.