If you’re creating a child theme: The template file you’re looking for is wp-content/themes/yourtheme/registration/activate.php
If you can’t find it in your theme folder then you need to copy it from wp-content/plugins/buddypress/bp-themes/bp-default/registration/activate.php and copy it into your theme folder. Remember to paste it into wp-content/themes/yourtheme/registration/activate.php
NB! Where ever I’ve written “yourtheme” replace it with the name of your theme’s folder.