For testing purposes, make this the first step of your script:
Show Custom Dialog [ Get ( AccountName ) ]
Then log in under [Guest] and check to see what is shown in this custom dialog.
This will confirm:
a) That the above script is the actual script performed when the file opens.
b) the exact text returned by Get ( accountName ) when a guest logs in to be sure that you have the correct text in quotes and that the file has really been opened with the [Guest] account.