Probably the account you are using to log into our nworks UI, does not have permissions to the registry on the nworks Management Center server.
Can you log into the nworks UI as a user with Admin permissions on the Management Center server? Then license import should succeed.
Note - we have changed this behaviour in v5.5, so that the Management Center service account will be used. When you upgrade to 5.5 you won't have this problem. There is an RC of 5.5 available on the website now
Please also note - you cannot "add" licenses to nworks. If you have purchased another socket, then please ask Veeam sales to issue you with ONE license file, that contains ALL your purchased sockets. Then you import that into the nworks UI. The Management Center only holds one license file at one time.
Hope that helps! Let us know,