This issue is still live and easy to reproduce, at least for me. I have the same thing, with this namespace and also with DataRowExtensions; in fact they do not automatically get imported or even pop up when I start typing System.Data into the Imports field.
Interestingly enough all the functions do work; the validation warning icon will disappear once you have the syntax correct, you just have to manage without the nice intellisense prompt.
Just as a sidenote - System.Data.DataSetExtensions is the library file name. Methods that it contains actually are in System.Data namespace (which is already present by default), so there’s nothing to add from Imports panel as it’s already there.
This is what might actually be confusing the Studio, as it’s not adding a namespace, it’s extending it.
Indeed, that’s why I suggested just typing it in. For me Intellisense catches on after using it and restarting the Studio - seems it doesn’t support hot-reload.
If it would not be working at all, you can also check directly in the xaml for assembly reference to that dll (and add it manually if needed).
Good point, I forgot about this and I think you’re right. This must be the crucial difference between these extensions and others, like those in System.Linq, that work just fine out of the box.