Is it posible to edit an XSD file? ObjectStructural.xsd for instance is not in the DataGroup Definition Editor it is the only place i know where to edit these files but i have no idea how to put it there.
Possible but editting ABD system schemas is NOT recommend, and system schemas are typically required in order for proper operation of the application or may be required in one of the many import/export, reporting, publishing and/or other interoperablity operations available in ABD. To serve you better, it would help us to understand what you want to change in the system and why.
Some schema options can be changed at the dataset level and would be the prefered place to make the change, since updates, new versions etc.. may automatically restore or update your schema to delivered specifications.
Vince, I'm also curious about the change you wanted to make in that particular file. I'm assuming you meant ObjectStructuralUsage.xsd, which simply contains a Yes/No boolean to determine whether an object is Structural in nature, and if so, it's applicable Structural material. This is why there's an accompanying ObjectStructuralUsage_enumlookups.xml file which stores the values for those two properties.
As Jeff mentioned, modifying system level DataGroup schemas can potentially cause more harm than good, depending on what it is that you modifiy.