If you are attempting to download MT-Notifier and you receive an error message that will not allow you to download the plugin (perhaps transaction not authenticated), the likely cause is an update to the software that handles the payment processing from PayPal. Unfortunately, the software needed to be upgraded due to a change at PayPal. But this broke the way that payments were handled previously.
Luckily, you can still use your prior purchases. You just need to make a small change to the link. Where previously the link included a script named linklokipn.php, change the script name to linklokipn-legacy.php. This also applies if you have a link with "manual" in the url AND receive an error - just change linklokipn-manual.php to linklokipn-legacy.php.
Do not make any other changes - specifically, do not change your authorization code, as that's what will determine what to download. Also, don't make the change if you don't get an error - because this version will only work with earlier licensees, and you will get an error with that version if you are a recent licensee. If that's the case and you changed it, you'll want to remove "legacy" instead!
Finally, don't worry about the filename at the end - even if it is an older version than you expect (say, 4.1.1 instead of 4.2.1), it will be fine. It is the authorization code that does the magic. Just make sure you add the -legacy bit in there and you'll be set.
If this doesn't work for you, let me know by providing your full download link, and I'll update it with your new download link in return.