I checked this on a test install.
If you get this message when installing the Addon:
"The files associated with this add-on could not be found. Please upload them and try again."
...it's definitely related to how/where the files were uploaded. I can't produce this error any other way.
In the Widget Addon (latest) pack, for each folder:
./library/ = 78 files & 36 Folders
./js/ = 14 files & 2 Folders
You may wan to double check that the exact count is uploaded to your server via your FTP program or whatever transfer method you use.
In a normal circumstance, I don't get any other error after uploading the files and running the installer XML file.