I reinstalled K2 but every time I try to create a new category I get this:

Warning: require_once(/home/lapirino/public_html/administrator/components/com_finder/helpers/indexer/adapter.php) [function.require-once]: failed to open stream: No such file or directory in /home/lapirino/public_html/plugins/finder/k2/k2.php on line 15

Fatal error: require_once() [function.require]: Failed opening required '/home/lapirino/public_html/administrator/components/com_finder/helpers/indexer/adapter.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/lapirino/public_html/plugins/finder/k2/k2.php on line 15 :(

It looks like you are having multiple problems with your k2 install.
Back up your site (akeebabackup.com)
check the information tab in the k2 dashboard, make sure the requirments for your server are met (multibyte encoding)
reinstall k2 and that may fix your errors
If you cannot get it to work, check my profile and email me access and i will take a look when time permits

First of all I thank you for your time William, I just upgraded to joomla 2.5.4 and reinstalled k2 2.5.5 with success but now I´m getting this error:

Fatal error: Call to undefined function mb_regex_encoding() in /home/lapirino/public_html/administrator/components/com_k2/tables/k2category.php on line 93

I included a snapshot of k2 information page, according to it, everything is ok.

This is how I fixed the issue.

The affected files are "k2category.php" and "k2item.php" in .../administrator/components/com_k2/tables/

and "route.php" in .../components/com_k2/helpers/.

Modifications - commenting out the lines with mb_regex and mb_ereg... as shown...

@ route.php, lines 56 & 59:
mb_internal_encoding("UTF-8");
// mb_regex_encoding("UTF-8");
$alias = trim(mb_strtolower($user->name));
$alias = str_replace('-', ' ', $alias);
// $alias = mb_ereg_replace(':space:+', ' ', $alias);


@ k2category.php, lines 50 & 53:
mb_internal_encoding("UTF-8");
// mb_regex_encoding("UTF-8");
$this->alias = trim(mb_strtolower($this->alias));
$this->alias = str_replace('-', ' ', $this->alias);
// $this->alias = mb_ereg_replace(':space:+', ' ', $this->alias);

@ k2item.php, lines 75 & 78:
mb_internal_encoding("UTF-8");
// mb_regex_encoding("UTF-8");
$this->alias = trim(mb_strtolower($this->alias));
$this->alias = str_replace('-', ' ', $this->alias);
// $this->alias = mb_ereg_replace(':space:+', ' ', $this->alias);