The following technique may work for you:
a. Add a file of any type (file can be empty and/or Hidden) in the same folder as the Excel file in Dropbox.
b. Exit if the file does not exist.
Sample code follows (tested and working):
It is a best practice to declare all variables. If you misspell a variable in your code, VBA will silently assume it is a Variant variable and go on executing with no clue to you that you have a bug. Go to the VBA development window, click Tools, Options, and check "Require Variable Declaration." This will insert the following line at the top of all new modules:
This option requires all variables to be declared and will give a compiler error for undeclared variables.
I hope this helps.