If the instructions from the developer was exactly those, it sounds to me that this data.txt file will be inside the folder that gets created when you unzip the file from him. If he included it, it should be in this folder that he asks you to upload to the server.
If not, might mean that you need to create data.txt yourself on your own computer and upload it to the the folder specified by the developer.
To do the latter, please
- open notepad
- without typing anything into the notepad, save it as data.txt
- upload it to the folder the developer told you to.
- then set the permission of the file to rwx-rwx-rwx (or 777).