When you have (access to) more than one brand, you can easily switch between brands in the top right corner of the screen. Next to your name, you'll see the name of the brand you're currently in. When you hover your mouse over the name, a drop down menu shows all your available brands. By clicking on a brand Media Injection will send you to the main dashboard of that brand.
If no brand is accessible your screen will remain blank, preventing you from interacting at all. Hunt down your administrator(s) to open up access to one or more brands to your account.