Depending on how long ago you purchased your BeagleBone Black, you may not have the latest version of Angstrom installed. It's usually preferable to get the latest version, especially this early in the product's lifecycle.
To start with, download the latest version of Angstrom for the BeagleBone Black. You can find the latest version at the BeagleBoard Latest Images page. Once you've downloaded the file, you'll want to jump to the section for your specific OS (Mac OS X, Linux, Windows).
The default login user/password for angstrom is "root". There is no password set, so you can bypass the password prompt by just hitting enter without typing anything in.
Angstrom comes with the Avahi Daemon pre-installed. This is really useful because you can now ssh in simply by using 'beaglebone.local' instead of trying to find the IP address. For example, type this into your favorite terminal: