Can't Bulk Upload Customer CSV File

Hello,
I hope someone can help me, as I've been at this for 2 days, I've been searching these posts, but cannot figure it out.

I am setting up a new website using Zen Cart 1.3.9 and have installed the Add Customer from Admin 1.07 add-on.

I have managed to get the option to add a single customer working by fiddling with the file permissions, but cannot get the bulk upload to work. Each time I try I get "There were Errors/Error Uploading File" message. Also, when I try to click on the "Formatting the CSV" link next to the upload button, I get a Firefox "File Not Found" error.

Altough I'm really new at this, it appears to me that the program isn't able to find the necessary files, although they all appear to be in the proper places. I contacted our hosting company to see if there was some sort of block on uploading scripts, but they said there wasn't. Also setting all the file permissions to 777 didn't work either, that only made it so I couldn't even get into the Customer section of admin, all I'd get would be the Firefox "File Not Found" error, so I changed the permissions back.

Maybe it just doesn't work with version 1.3.9?

I'm at a complete loss at this point, so any help would really be appreciated.