Please set connection with your server using your FTP client or just make sure it is already connected. Then go to the root directory with the theme.
Click on the folder "timthumb_cache" right mouse button and select settings "Change permissions".
In the pop-up window of the "Change permissions" please select the settings "Octal notation" 777 as the access rights to the folder. Don't forget to put a tick "Process selected directories recursively". And click "Change" button.