You may also need to change the type/creator codes of the .iso file you've downloaded so that Toast can recognise the file correctly.
For Toast 4.1 the correct type/creator codes for an ISO image are:
Creator: CDr3
Type: iImg
Use a program like DropConversion or SuperGetInfo to make the change.
Once you've done that, you'll be able to just double-click the file and Toast will open and set things up properly to make the burn.