The first way (Recommended): You can import the Decal pack from the preferences of the DECALmachine add-on in Blender

The second way: You can unzip the .zip in the folder of your decals (if you don't know which folder is it, check in the preferences of the add-on) then click on the refresh button to see the pack appear in the list 

Bug with textures?

If you get any problem with the decals not showing use the Texture Fixer tool in DECALmachine

Yes