Unless you created it as a temporary partition in the Utilities menu the .toast file is an image file that is the same size as its content so there isn't any space available inside to add anything.
I don't know what you have in the .toast file but an easy way to add more is to mount it, drag the contents to the appropriate format in the Data window and add your additional folders there. At the point you can burn the DVD.
If the .toast file contains a VIDEO_TS folder then choose DVD-ROM (UDF) as the format in the Data window. If it only contains other data files such as the images you want to add then you can choose most any of the formats.
To mount a .toast file you can control-click on it in the Finder and choose Mount It from the contextual menu. Or you can select it in the Toast Copy window with Image File as the setting and it will mount.
In the future you can choose Create Temporary Partition in the Utilities menu to create a .toast file of any size (such as one that fits a DVD). When you quit Toast the first time after creating it you'll be asked if you want to keep or delete the temporary partition. Choose to keep it. From then on you can mount the .toast file and keep adding content to it until it gets full. You burn it using the Image File setting in the Copy window.
The toast file was one I made off of the dvd we were given for my soon to be born son's imaging session today at lunch. The dvd was made by a Sony dvd recorder and the format wasnt letting me see the Video_TS folder (or at least Im a little too brain fried to figure it all out at the moment). I can make a new toast file easy enough, I am just wanting to add the stills files onto the dvd disc....for some odd reason they put it on a cd. Neither disc is anywhere near capacity. I will try making a new toast file with the Temp Partition now and see if I dont screw that up.
Thanks for the suggestion....I knew was missing something and had the feeling it was kinda obvious.