I don't know of any red hexes. But maybe you mean "toggle hex view," which would be the "hex" button in the bottom left corner?
/Edit: Or maybe you want to place blockers, which aren't "the red hexes," though. You must go to Tools -> Toggle Block Obj View, then you can see everything. Now you have to search the respective blocker in the scenery tab or wall tab, if you want to add wall blockers. Alternative is to use Mash's High-Res patch for the mappre and press the TBOV button, then you don't need to go the long way via Tools -> Bla bla.