If you've registered for an application, you will have to wait for the details to come to your email to be able to activate the account. In case you've used an online service, the activation process might take a while. It depends on how the developers are working. Some programs/services don't offer the possibility to use them after you've just registered as an activation procedure needs to be done. Additionally, check your email and confirm your account, because this is also possible when it comes to program's accounts or online services.