NTFS for Mac OS X always installs as trial version. When trial period expires it stops mounting NTFS partitions.
To activate it to full version and continue using it please do the following:
You have to use exactly product key and serial number that you bought from Paragon. If you want to use Paragon NTFS for Mac OS X as trial version, you don’t need to enter serial number anywhere.
This article refers to Paragon NTFS for Mac 9.0, 9.5 and 10.