To piggy-back on what Jean-MIchel said:
"When you started creating the backup specification, did you check the "Source-side deduplication" box in first screen, at the bottom? On an existing job, you have it in the properties, Options tab."
The integration Guide says, in the Creating Device Section
"Optionally, select Source-side deduplication to enable source-side deduplication. The Source-side gateway properties window opens. Review and if necessary modify the source-side gateway properties. By default, the source-side gateway will be named DeviceName_Source_side. Note that you can create only one source-side gateway per device. This (virtual) gateway will then be automatically expanded on the backed up system if source-side deduplication is enabled in the backup specification"
So, this is telling me that, if you defined Source-Side deDuplication in the Backup specification, you can't define it againwhen creating a Device, and the reason Source-side deDuplication may be greyed-out is that it has already been defined in the Backup specification