I am able to register the license. However, when I try to load the license using the following command:
LIC LOAD BUILDERXCESSORY

I get the error
%LICENSE-W-WRGARCH, BUILDERXCESSORY license is not valid on this architecture

When you install BX5.0 VMS license key on a previously installed BX3.5.1 VMS license key, the above error message is generated. This is because it has the same product name as the earlier one. You need to delete BX3.5.1 license key completely before installing BX5.0 license key.