I bought a theme some month before and want to use it now but can’t find a way to register it.
What do you mean by ‘register’ it?
This was from themeforest and not elements right?
Please check the theme documentation hopefully you will get necessary support in documentation as like how to install, how to active, how to register theme license etc. If you have still problem with the theme then The best way to get the support is Contact your purchased Theme Author. Contact Author and let them know.