I create a category, but below I see nothing. No category showing on the list.
I create an article, linked to the category (here I see the category), but on the list of article, I see nothing, no articles appear ????? why ?

If after creating the category or item, it does not show, clear your browzers cache and reload
Check with phpmyadmin to verify that the category and or item is either created or not

I did, but in firefox nothing appear. In safari yes, but I don't work whit safari.

Ok, I solved. In Firefox to clean well the cache you have to select 3 case: cache, cookies, access. Only cache case selected is not enough.

Thanks Jettero, this has fixed it for me too. It seems you have to delete the cache and clear the cookies each time you open a new browser session. This is annoying but better than not being able to use K2 at all!

Hopefully there will be a fix soon ;)