any way to spilt a long article into a few different page?

I'm building a magazine website and some of the articles a a bit too long. how can I add the feature next page?

Use page break button in editor.

http://docs.joomla.org/Splitting_a_long ... nked_pages

I've tried that when I read it online earlier but it doesn't seem to have the next page button. The only thing appeared is just one line as attached below.

Which editor are you using?

I've just remembered I'm using K2 articles for this. I've created another article in the article page and it works fine. How can I activate it in K2 articles?

Page breaks in K2 was always a problem. Joomla pagebreak plugin only works with joomla content.
If you are using JCE editor try the pagebreak button in JCE editor itself.

I'm not using JCE, just your usual K2 article settings. So I can't do page breaks for K2 articles. Do you know if there are any plugins to can work in a similar function.

Try below plugin,

http://extensions.joomla.org/extensions ... tabs/20479