I installed the last K2 (2.5.5) for Joomla 2.5 (2.5.3)
When i want to create a new category, if i click save i get a error:

Joomla!
500 - An error has occurred.

Table 'mercur_15.mercur17_finder_types' doesn't exist SQL=SELECT `id` FROM `mercur17_finder_types` WHERE `title` = 'K2 Item'

Return to Control Panel

I'm on localhost wamp+win 7:

PHP Built On Windows NT PENRYN 6.1 build 7601 (Windows 7 Home Premium Edition Service Pack 1) i586
Database Version 5.5.16-log
Database Collation utf8_general_ci
PHP Version 5.3.8
Web Server Apache/2.2.21 (Win32) PHP/5.3.8
WebServer to PHP Interface apache2handler
Joomla! Version Joomla! 2.5.3 Stable [ Ember ] 15-March-2012 14:00 GMT
Joomla! Platform Version Joomla Platform 11.4.0 Stable [ Brian Kernighan ] 03-Jan-2012 00:00 GMT
User Agent Mozilla/5.0 (Windows NT 6.1; rv:11.0) Gecko/20100101 Firefox/11.0

If you do not use the K2 Smart Search you can safely disable it to fix the problem.

joomla admin > Extensions > Plugin Manager > Disable "Smart Search - K2" plugin.