Hi , I have a k2 site , and i need to disable login requirement for adding and editting items , how can I disable login or register for guests? ( I am using joomla 2.5 and latest version of k2 )

I don't think this is possible by configuring K2 in any way.
This needs hacking K2 core files.

for doing this you should go to users > parameters > disable registration
This is a joomla paramter which is used to disable /enable registration of new users...
if this parameter is set to off, none can register in your site.
The new user should be manually inserted via the backend.