Are you referring to the create folder function within the webmail application? If so, I don't believe you can create a folder at the same level as Inbox. If you SSH to your site and then do an ls -Al ~/Maildir, you will see all of your folders directly under Maildir listed as .Folder. There will also be folders there for cur, new, and tmp. These three are your Inbox mail. Each of your mail Folders will also have cur, new, and tmp sub-folders containing the mail for that folder. So, essentially, the Maildir folder is your Inbox folder which is also your root mail folder and you can only have one each of cur, new, and tmp at that level.