I suppose you reinstalled it on the other PC or reinstalled Windows. Your registration information is stored in registry. And it is not deleted upon reinstalling. If you some how deleted that information, didn't copy registry keys needed and you were the only administrator in that network - than you can't. If there is another administrator - just ask him to give you admin rights. If there are no more administrators in that net - than the only possible way is to create another net and ask users to join there instead.
this informatiom is stored here
HKEY_LOCAL_MACHINE \ SOFTWARE \ Famatech \ RadminVPN \ 1.0 (for x86)
Or
HKEY_LOCAL_MACHINE \ SOFTWARE \ WOW6432Node \ Famatech \ RadminVPN \ 1.0 (for x64)