Installs Themes

hi,i permanantly use twenty ten themes for my present themes,i using free themes download from Artsteer website,and also i had do some change on my wp content subdirectory file;i had create a folder name it twenty eleven child and also i had include style.css in this folder,but when i upload my new themes to my wordpress,the error message “Unable to create directory D:\Hosting\xxxxxxx\html\wordpress/wp-content/uploads/2012/04.
Is its parent directory writable by the server?“shown up on my blank browser,can u help us solve this issue,what the next step further to figure up ?

thanks,
sunny