In Mac OS X you can apply your .icns easily.
First of all select your folder/file in Finder, then right click on it and select Get Info.
In the info panel, locate the folder/file preview in the top left corner.
Now drag&drop your .icns file in the preview. You can use Image2icon export feature for creating .icns from your images.
If your user does not have the right permissions, Mac OS X will ask you to authorize your action.
There you Go! Check here to return to your original icon.