You'll have to update DT Register to the current build to work with the current Joomla. Go to My Orders... not Invoices... and the download link will be there. However... if your order was over a year ago, the download will not be available and a new license purchase will be required.
Nathan is no longer affiliated with DTH since the recent acquisition. You can connect with him and get any type of Joomla website help at
www.JoomlaEmployee.com
.