You can tell Backup Exec to use software compression by going to the template (or job), and in the left pane under Settings, go to General. At the bottom of the right pane there is a drop-down box under Compression type that should be set to either Software or Hardware (if available, otherwise software). This will use the resources (memory and CPU) of the Backup Exec server to do the compression before the data is written to the backup-to-disk destination.