Super simple stuff. Why not wrap the content you want initially hidden in a div, slap an h3 or something in before it, and go something like this:

// Hide the extra content initially, using JS so that if JS is disabled, no problemo.
$('.read-more-content').addClass('hide');

// Set up the toggle.
$('.read-more-toggle').on('click', function() {
  $(this).next('.read-more-content').toggleClass('hide');
});

CodePen: http://codepen.io/JoshBlackwood/pen/tmpwq

Edited to add: After reading the link you posted, I think I may have misunderstood what you're wanting. If you want this to be inline, the concept is similar, and only slightly more complicated. You could wrap the content you want to be expanded in a span, and use JS to dynamically insert a 'read more' link. I'll fork my CodePen with an example, one sec.

Edit: Didn't see your reply. No problem, glad I could help!

Here's another example, of this done inline, more like the expander plugin but without the added dependency (or features). This is hacked together, and could certainly be done cleaner, just as a disclaimer.

http://codepen.io/JoshBlackwood/pen/tDsEh

Edited again: Cleaner version with no HTML injection, but this also removes a lot of the convenience of just adding a class to a span to handle everything for you: http://codepen.io/JoshBlackwood/pen/pEwHe

And how could I achieve that if I click on a closed div´s read more that the other opened div´s will close?

Unless there's something else going on, the url to the readmoreless.js-file is not /js/readmoreless.js, but is likelier to be along these lines: wp-content/themes/ThemeName/js/readmoreless.js, which might very well be what the issue is.