Assign a post/item to multiple category?

I've been using Wordpress for a very long time, and now with Joomla, I just can't find how to assign a post/item to multiple categories. My example is this one: I am writting an article about Call of Duty: Black Ops 2 and I would like to assign it to multiple cateogies such as: PC, Xbox 360, PS3. The editor allow let me click one single category, is there something I am doing wrong?

Thanks for your reply!

This is how it is designed in Jooomla You can always try to use K2 component to add your content and try this >> http://extensions.joomla.org/extensions/extension-specific/k2-extensions/17402

Any extension for joomla 3.0?

I don't know any ... try to write message to this plugin authors maybe they prepare J!3.0 version.