Please can you send us the link of the add-on from the add-ons site because we are not aware of a product of this name. We have one called Beauty Retouching Kit but not Beauty Retouch so it is possible that you have an old version of an add-on.
I bought this plug-in from retouch academy website. This is the link below.
who can I obtain a valid signature to install the plug-in
This is not an Adobe product so Adobe programs such as Extension Manager will not open the file. You must contact the source where you purchased the product from to find out how to install their product.