To get started with the Architect, configure one or more Database
Connections (right-click in the Navigator panel and choose New
Connection...).  You may need to add one or more JDBC drivers; this
is done under File->User Preferences->JDBC Drivers.

Then, add your Connection to the Project and expand the database
in the navigator; drag the parts you want to work on into the main
panel. You can now begin visually inspecting/editing your database.
Save your Project if you want to resume it later.

Note: If you have used a version prior to 1.0 you may need to 
re-configure your JDBC drivers and/or connections.