Your todo.txt file should be placed in a "todo" folder in Dropbox. More Information -> Gina Trapani: http://todotxt.com
If
Every day of the week at
This Trigger fires only on specific days of the week at the time you provide.
Then
Append to a text file
This Action will append to a text file as determined by the file name and folder path you specify. Once a file’s size reaches 2MB a new file will be created.
Explore more great ways to automate Dropbox and Date & Time