If you haven't already, start SourceTree. Then, do the following:

• Choose View > Show Hosted Projects from the menu bar.
  The system displays the Hosted Repositories dialog.
• Press Edit Accounts.
  The system displays a list of the hosted accounts it knows about. If this is the first time you have run this, the list is empty.
• Press Add Account.
• Complete the dialog:

• The Username is your account name. The password you provide is your account password. You can choose whether you prefer connecting through HTTPS or SSH.
• Press OK.
  SourceTree returns you to the list of hosted accounts. You can add another if you like.
• Press Close when you are done adding accounts.
  The system displays a list of your hosted repositories and repositories your account is following.

• Press Close.

You'll notice that the SourceTree does not list your hosted repositories on your Bookmarks list. The bookmarks list is only for local repositories; you'll need to clone from a hosted repository or create a local repository to add to this list.