For a clue on what the path should be.
Assuming you have a working forum, goto
Control Panel » DB, Paths & URLs
copy Absolute PATH to Main Directory: to your
Control Panel » Feature Settings-Attachments
Attachment Storage Directory: and adjust accordingly to where your desired attachment directory will be at.
Then make sure your attachment directory is set to 755 using your ftp program.
Then run the test link.
If successful it will place a file called "test.file" in the directory. This file is needed to make the feature work.
