Here's what I use:
Attachment Storage Directory: /home/mysite/public_html/forums/attachments
Full URL to Attachment Storage Directory: http://www.mysite.com/forums/attachmentsIn
CP > Feature Settings > Attachments there is a link to check that the directory you specify works correctly.
On the server side (FTP) enter your forum directory and, in your case, create a new directory called
uploads the CHMOD it to 777.