To connect to a CIFS share under Mac OS X, Select Go > Connect to Server (Cmd-K) from Finder, then type "smb://filer/sharename" in the "Server Address" field. Replace filer and sharename with the appropriate DNS name (or IP) and share name.
When you're done, find the host under the "Shared" heading on the left column of a Finder window, and click the eject button.
You can change the root directory of the HTTP server to be whatever you like. However, it's limited to one directory (and it can't be just /vol). If you need more paths, then the current recommendation is to put a full FTP server in front of the storage and allow it to access the data via the standard protocols (usually NAS).
From what I can tell, xftp is just a specific ftp client. While I don't have experience with it specifically, I can't think of any reason off the top of my head why it wouldn't work.
There is no sftp support currently in Data ONTAP. I believe there is already an RFE filed and you're welcome to call into support and have your account added to the RFE, but if you require sftp support for file transfers then you'd need to use a full sftp server.