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.
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).
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: