How can I extract/use icons in an icontainer file?

Solution 1:

You need specific software to open .icontainer files, e.g. CandyBar.

To use an iContainer, simply double-click it in the Finder. If it is not already running, CandyBar will launch and automatically create a new collection in your icon library. You can also import iContainers by simply dragging and dropping the iContainer directly into CandyBar's List Pane. If you would like to view the contents of an iContainer before importing it into CandyBar, and you are running Mac OS X 10.5 or later, simply select the iContainer in the Finder and press the spacebar. This will activate OS X's Quicklook feature and you can browse the icons within the iContainer quickly and easily.

Solution 2:

Have you tried out LiteIcon? It should be able to open iContainer format packages, and it is a free alternative to CandyBar.