WP will not allow me to upload a header

Hi
I have recently started putting together my website. I have created a header and want to upload it as I have for another site. The trouble is I am getting the message “The wp-content folder is not writeable by the server” and there is no Browse button to find a file with.
Is there something obvious that I am missing? Is it because I only just set up my domain to point to my web hoster’s servers yesterday and something need to run its course? I would be grateful for any thoughts.
Many thanks in anticipation.
Caroline