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).