If your using Transmit, then use the iDisk built in preset and you just need your username and password.
Alternatively you can pick webDAV. The webDAV server I pretty sure is just idisk.mac.com, but I'll check.
(The iDisk preset is of course using webDAV. It is many times faster than Finder, because Apple have the slowest implementation of the webDAV protocol anywhere, with many redundant, unnecessary calls).
I don't use FTP to transfer files to MobileMe anymore. Make sure your iDisk is showing under Devices in finder and just drag the files you want to which ever folder you want on iDesk and drop. I move whole websites that way.
Been trying to upload some work to my brothers mobme public...why such a problem with largeish zips (300+megs) when using web interface? it just fails instantly - anyway thanks transmission says its doing it!