How can I show/hide a specific alert with twitter bootstrap?

On top of all the previous answers, dont forget to hide your alert before using it with a simple style="display:none;"

<div class="alert alert-success" id="passwordsNoMatchRegister" role="alert" style="display:none;" >Message of the Alert</div>

Then use either:

$('#passwordsNoMatchRegister').show();

$('#passwordsNoMatchRegister').fadeIn();

$('#passwordsNoMatchRegister').slideDown();

You need to use an id selector:

 //show
 $('#passwordsNoMatchRegister').show();
 //hide
 $('#passwordsNoMatchRegister').hide();

# is an id selector and passwordsNoMatchRegister is the id of the div.

Add the "collapse" class to the alert div and the alert will be "collapsed" (hidden) by default. You can still call it using "show"

<div class="alert alert-error collapse" role="alert" id="passwordsNoMatchRegister">
  <span>
  <p>Looks like the passwords you entered don't match!</p>
  </span>
</div>