I can't see the rectangle box but it sound like the rectangle box is behind the tab control, so you would need to move the tab control to the back. In edit mode right click on the tab control and then click on arrange then click send to back.
You might also try this trick:
1) enter layout mode
2) click the tab control to select it.
3) note the top and left position in pixels on the inspector's position tab
4) drag the tab control to another spot on your layout
5) now that you have a clear field of operations, select and delete the rectangle.
6) Select the Tabl control by clicking it
7) enter the original values for top and left into the top and left boxes and it will pop back into it's exact original position.