No box shows up because you are picking the point before you start the group placement. Why not try combining 2 lines into a single line of code:
Instead of:
XYZ targetPoint = Command.uidoc.Selection.PickPoint("Please pick a point.");
and;
Group dt= uidoc.Document.Create.PlaceGroup(targetPoint, ty);
try something like this:
Group dt= uidoc.Document.Create.PlaceGroup(Command.uidoc.Selection.PickPoint("Please pick a point."), ty);