When trying to add an addon for the first time I got this message:
"The files associated with this add-on could not be found. Please upload them and try again."
It seemed I had followed the instructions correctly, uploaded files to the correct folder(s) etc. After searching the xf forums it seems quite a few other people have experienced the same problem at some time.
The resolution! (at least it was for me.)
By default I have my FTP set to force all uploaded files to lower case. The reason the addon files could not be found was that they are in mixed case. After turning off the 'force lowercase' option on my FTP, and uploading, voila, all works!
I hope it saves someone a bit of frustration!
XF: Please consider expanding the error message to say:
"The files associated with this add-on could not be found. Please upload them and try again. Check that the uploaded file names upper/lower casing has been preserved in case the addon requires it."