Hi Victoria,
First, if the sandbox has expired please Activate a new one.
When you click the link in the exercise to navigate to the Azure portal, the portal should open and you should see MICROSOFT LEARN SANDBOX in upper right corner along with your account name. If not, please click the gear icon and click Switch button next to sandbox.
If the above doesn't help, please post screenshot of what you are seeing when you navigate to the portal. You can block out your account name in upper right for privacy reasons.
-TP