If you have registered a major version of VSdocman older than the latest one, your registration key will not work with the latest version. Your registration key is only valid for one major version. If you for example purchased version 5.x, the key will not work with version 6.x.
For the latest major version you need to upgrade. If you don't want to upgrade, you can download the latest version of VSdocman which will work with your registration key.