Two things need to happen to make a dashboard your default:
To be performed by an Admin User
In the top left, use the dashboard drop-down and select Manage Dashboards
Search for the required dashboard and click into the record
Select the Default Dashboard check box.
Click Save
To be done on a user by user basis
In the top left, use the dashboard drop-down and select Manage Dashboards
Search for the required dashboard, but do not click into the record.
Next to the name, click the star icon. This will mark it as your favourite.