When you first login to Brandcast, you'll be taken to your My Brandcast dashboard. Here you'll see a quick view of the 5 sites you've most recently worked on. If the site you want to open is here, hover over it and click to open.
If you don't see the site you want to open, you can click on Websites in the left column, or on the gray "View all websites" button in the upper right of the My Websites section.
On the next screen, you can view all websites you have access to. You can search for a specific one using the search bar, sort by name or date created, and you can filter to see similarly tagged websites. If you hover over a thumbnail you can open the site by clicking on it.
After selecting your website, you'll be taken to the website-specific dashboard where you can choose what page to open to edit.