What version of OS X are you using? What kind of Macintosh computer do you have? What version of Backup are you using? How are you connected to the internet? Has Backup ever worked to your iDisk, or is this the first time you've tried to use it? Can you access your iDisk OK?
Please elaborate on your situation greatly. "Backup to iDisk won't work" is extremely vague, and, as such, we can only offer equally vague solutions.
I'm running Mac OS X version 10.4.2 on a G4 15" Powerbook running at 1.67 GHz with 512KB L2 Cache, 1GB Memory and ROM version 4.9.1f1. I am using Backup 2, version 2.0.2 (v174.5). I am connected to the internet through a WiFi network using AirPort. Backup to my iDisk has worked in the past, but fails every time now; it is set to back up daily. When I try to access my Idisk through Finder I get the message 'The volume for iDisk cannot be found.