Hit the normal menu button on your phone. I have a Samsung Galaxy S and it is the one on the bottom left. This should bring up a bottom menu bar with "Home," "Settings," and "Support."
This will bring up your Mobage account page. Scroll down to the bottom and "log out."
After you log out, RoB will close. Reopen the program. You will get a Mobage log in screen. Just go through the process of creating another account.
If you leave it like this, you will have a second account that you can use for feeder cards, but it will not have a terminal registration (you already have that for your main account). For some people, that is fine. If you want to get all of those presents that come with terminal registration, you need to bum someone's phone that will not be playing the game. Load the game on their phone, log in to your account, and then perform a terminal registration. After that you can switch back and forth between your two accounts on your phone and both accounts will be registered.