Technet Online says:
To view or configure the backup compression default option
In Object Explorer, right-click a server and select Properties.
Click the Database settings node.
Under Backup and restore, Compress backup shows the current setting of the backup compression default option. This setting determines the server-level default for compressing backups, as follows:
If the Compress backup box is blank, new backups are uncompressed by default.
If the Compress backup box is checked, new backups are compressed by default.
If you are a member of the sysadmin or serveradmin fixed server role, you can also change the default setting by clicking the Compress backup box.