CrashPlan always runs in the background automatically whenever your computer is booted up. You can access the CrashPlan application from the tray icon. In Windows this is located at the bottom-right of your screen as shown.
If you are using Mac OS X you can find the icon at the top-right of your screen as shown.
From here you can click the cog icon to show the menu, and this will let you open the application. You can also choose to pause your backups and access your backup options.