Assuming that this is a public folder and not simply a shared folder from a user's mailbox, you'll need two things. First, the user setting up the rules needs have Owner permissions for the folder. Second, the user needs to have Send As permissions for the folder.

With both those permissions, you should be able to right-click the public folder and go to the Properties. You then want to click on the Administration tab and click on Folder Assistant. You should be able to create the new rules there.

Unfortunately I've found that public folder rules aren't entirely reliable.

-SW