I presume the serial number must've come in an email or something. Anyway I've uploaded the skin pack to my Microsoft one drive. You should be able to install it without the serial as the serial is really just for accessing the skins from Birdline website.
At first I was like you could not find independence pro folder... But then I turned the file manager to view "hidden folder and files" and saw the "ProgramData" folder (Windows 10-home), if you can launch Independence from samplitude then you must have this folder lest you changed the default folder somewhere else. ... It took me awhile to find it but its there... It should look something like this: "C:\ProgramData\MAGIX\Independence\VST Plug-Ins"