Some users complain that they don;t see the message that they need to activate their accounts. I wonder if there is any option to keep the message there until user close it, or create landing page after they click to register?

Keep them open also after reload is not possible.
Keep them open on same page is possibke by removing piece of code from global.js

Ok thank you very much. I edited the code, which i paste below so other people can locate it easily too, if they want

// HIDE FLASH MESSAGES AUTOMATICALLY
  window.setTimeout(function(){
    $('.flash-wrap .flashmessage:not(.js)').css('opacity', 1).css('margin-top', '0px').css('margin-bottom', '0px').animate( { opacity: 0, marginTop:'30px', marginBottom:'-30px'}, 300);

    window.setTimeout(function() {
      $('.flash-wrap .flashmessage:not(.js)').remove();
    }, 300);
  }, 10000);
I edited the 10000 miliseconds...
(i think thats is right, right?)

Which file is this? where you adjust the code
.

It's global.js in theme folder/js