I cannot select the second tab out of two tabs why?

HI,
I have added two tab Items to a tab control and have added a Grid to the first tab Item. I want to add another Grid to the
second tab item but i can add this by XAML window but at design time i cannot select the second tab to see the Grid and it's layout (as in windows OLD forms tabs)... why is this? and how can i fix this?
cheers