You need to copy the file onto the computer (drag it to the desktop/a folder) to create an editable source. Then when you want to save it back again, go into toast, select the CDRW an place the file back onto it (I suggest a slight name change in the revised edition, such as
mydoc to
mydoc1 so you can save both versions), selecting burn when you finish. Very easy.
Note: This can all also be done from the desktop if the CDRW shows up on the desktop as a CD shaped volume. If it does mount like that, ignore the previous instructions and just drap and drop files onto it and ejecting the disk will automatically bring up the option to burn the data onto the disk.
Its safe to say that since you said youre using toast titanium that you are using a mac running OSX.