its is a eclipse perspective so you don't need to start it just click on the >> button on the topmost right corner you can see other perspective select DDMS there DDMS perspective will open if you want to switch back click on java perspective. If you can't see DDMS option there just go to Windows->openPerspective->others and from the dialog box select DDMS.
To activate DDMS you need to run your application in debug mode and then click on ddms you will see all options once your application started.
hope this help
