Help me with installing or selecting a template

[dulop]

Hello!
When i try to change a template by any of default themes (green, elements, or main), the page gives me a error:

Error. The specified action is invalid or does not exist!

Or when i try to install a new theme, the page says:

Theme Location Error
The theme you are trying to install cannot be found on this server at the specified URL. Please go back and double check that the path to the theme's HTML file is correct. If you uploaded the theme and you are sure the URL is correct please make sure your web host does not have PHP's safe_mode configuration enabled as this can interfere with using or locating themes.

¿What is the problem? i have been trying for hours but i dont understand what is the problem :/

P.D.: Sorry for my bad english, i am not native speaker.

Thanks!