Check if files exist in a formula

I have been getting help on the MS forum about how to check a folder for files with a particular name. So far I have go to this:

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When a file is found, the script moves to the next file. But when one is not present in a folder, the macro stops. Rather than stopping/crashing, I would like a message box to show with a message e.g. "WORKBOOKNINE.xls Not Found". Then once the user clicks ok it will look for the next file.

Can anyone help?

Cheers
dvent

Result:

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