The sysbk program does not come with an automated restore tool (If you want automation, use the Reseller Level → Manage User Backups, or Admin Level → Admin Level Backup).
System Backups are stored into custom paths that the admin can specify. An example: /backup/07-30-04
where the numbers represent the date of that backup under the /backup folder.
If (for example) you want to restore your /etc/virtual directory, you'd type:
cd /etc tar xvzfp /backup/07-30-04/custom/etc/virtual.tar.gz
A list of files/directories used by DirectAdmin: directadmin.com/paths.html
Be careful with the /etc/passwd, /etc/shadow, /etc/group, etc.. files. Test out your login/lougout of ssh with another window before logging out, after replacing those files to ensure your login still works.