Marc-Antoine,
the DATA_GRID property is still available as far as I'm aware, but it's been a long time since I looked into it. IIRC, it's currently not available because Designer now operates directly on the BPMN model from the XML, which doesn't provide a straightforward way for storing the value of the complex field and reloading it when you re-open the file.
In the old setup we had for Designer's model the property could be added and specified as described in the userguide. It should have been stored and loaded from the BPMN's underlying model properly. However, since the data grid is variable by nature as to what you use it for, we wouldn't export anything to the BPMN XML file. We simply didn't have a way to determine how it would be used in the service task's delegate. For this, you had to write a custom export marshaller.
So, at the moment, it's not usable in Designer I'm afraid.
