You will need to start up the 'site' application first. The 'site' application ant task 'jetty-demo' has a dependency on the ant task called 'start-db'. This ant task will start up an HSQL (in-memory) database that the admin can then connect to. Also, since the Hibernate startup in development.properties is set to 'create' then it will also import a lot of sample data that you can play around with (see the scripts in the 'core' project).
Alternatively, you could follow the docs to switch to MySQL server if you don't want to use the embedded HSQL database: http://docs.broadleafcommerce.org/core/current/tutorials/getting-started-tutorials/switch-to-mysql-tutorial.
Basically, you have a few options here but the easiest solution is to start up the site application and have it running alongside the admin application. Then you won't see that startup error in the admin.