To add a database schema snapshot to the workspace click on the "Add Snapshot" item under the Database menu.
On the "Add Database Snapshot" dialog window that appears click on the "Browse" button to browse to the location where the snapshot(s) you want to add to the workspace have been stored. You can select as many snapshots as you want from different locations. For each snapshot you select the application will display the name, the date the snapshot was taken, the Database the snapshot was taken from, the SQL Server where the database was when the snapshot was taken and the SQL Server version.
By default the "Add" checkbox is checked for all the snapshots you have picked - you can uncheck the snapshots you don't need and keep just the ones you want to add to the workspace.