To use OGBB, the only thing you need is to download and install the application, run it, and create a new user account. From that moment on, you can use two accounts on the same device – one from the official BBM application, and the other from OGBB.
Besides the ability to manage two accounts, OGBB doesn't do anything else. This means if you aren't thinking about using two different accounts on the same device, then you won't really have any use for this alternative client. If, on the contrary, you want to use two accounts simultaneously, then this is the app you're looking for.