To find out how, read the full hint.
Before OS X, you could drag a folder with a custom icon into an empty data CD window in Toast and that folder's custom icon would become the CD's custom icon. This doesn't work anymore with OS X. What you need to do now is to save your custom icon as a .icns file (using a tool like Iconographer or Icon Composer) and drag it into your Toast window, placing it at the root level of your new data CD. Then rename the file as
.VolumeIcon.icnsOnce you burn the CD and pop it into your CD-ROM drive, it should appear in the Finder with a custom icon. :)
Background: Pre-OS X versions of MacOS store a custom icon for a CD (or any other type of volume) the same way they store the cutom icon for a folder--in an invisible file called Icon. OS X still recognises and displays the Icon file for folders, but not for volumes. In OS X, the custom icon for a volume is stored in an invisible file called .VolumeIcon.icns at the root level of the volume. The OS X Finder knows this and will create this file when you paste onto the info window for a volume and creates a .VolumeIcon.icns file. Toast 5 doesn't do this automatically, hence the need for this hint.
Note: This is all based on observation (not any official/unofficial documentation), but it seems to hold true.

