Any Boards that you create or which are shared with you will be stored within your My Boards area. You can select a Board to view or to make changes to it by selecting 'My Boards' and then the Board you wish to access.
How to select a Board
With the Boards section selected, click 'My Boards' You'll find this option in the top left of the currently selected Board:
Then simply select the Folder or Board you wish to access: