Hello there! Sorry for late reply.
That issue is popping up more and more - it seems that a recent change in wordpress or something else associated with wordpress has cut that directory off from public access, which is required for the uploader. It seems random who it affects. It got me too. The same issue got in the way of paypal's IPN as well (but that has been fixed).
I intend to move the upload directory into the wp-content folder in a future release,
but for now simply use an FTP program like filezilla, and use the path specified. By the way, welcome to the Symbiostock project