His 5.1 system has to support DD+ for that...If it's an old box, it may not have support for DD+...
@Velu61, if your home theater supports Dolby Digital Plus (DD+), select that on the Xstream box...If your 5.1 system supports upto DD/DTS, you can select Dolby Digital because in that case, even if you select DD+, you won't get that out of your 5.1 system if your system doesn't support DD+...