use a ftp software (or cpanel) to change the upload folder permission(s). Navigate to httpdocs/wp-content/uploads and set the folder permission (including the sub folders) to 777. Alternatively I'd suggest to unzip the package and to upload the theme folder by using a ftp software like filezilla to the wp-content/themes folder.
Please unzip the package you got from themeforest.net ONE time. You should get a folder which contains PSDs, Docs and another zip file (which is called cubit.zip. This zip file is the right file. Go to your wordpress admin panel and navigate to Appearance > Themes. Then select the cubit.zip file, upload & install it. You can also unpack this zip file and upload the resulting folder via ftp to wp-content/themes.
Also have a look at this tutorial video @themeforest.net: http://wiki.envato.com/buying/support-buying/solving-broken-theme-issues/