I don't understand why I can't access the below input text field using the next code of jQuery, in this case for example to change its "class" attribute: (through the "id" attribute?):

jQuery("#jform_nombres").attr("class", "inputbox ");

<input type="text" id="jform_nombres" name="jform[nombres]" class="inputbox validate[required,custom[stringaz]]" value="" size="32"/>

I'm stuck with this, thanks for any help!

To which file are you adding this?
Where is it loaded - onbody or head?
Does your script initialise on document.ready?
Do you have any JS alerts?
Do you have jQuery loaded?

If jQuery is loaded, try setting a local var in your script
If you get the result you're expecting then try using the prop() method in the same way you use attr().

Hope it helps!

Gez