How do I use a power adapter extension cable? So I got this strange thing here: http://www.apple.com/shop/product/MK122LL/A/power-adapter-extension-cable

It says that


  The Power Adapter Extension Cable is an AC extension cord that provides extra length for your Apple power adapter. Use it with MagSafe and MagSafe 2 power adapters and 10W, 12W, and 29W USB power adapters.


So I guess I can use it to extend the length of the MagSafe power cable? But how? I see no holes that fit it!

Also, I got this:



Which looks like the extension but without the cable part. Is this related?
The picture in your question is referred to as an AC wall adapter, or "duckhead" (US) and slides out of the AC power adapter so you can then slide in the AC power cable (US), pictured in the link in your question.

The picture below shows the "duck head" being slid out of the AC power adapter and once removed the AC power cable gets slid in its place, thus providing an extension cord to the AC power adapter.