It is not currently possible to use the OverlayOptimizer on your OptimizePress blog pages or posts (you will not see it in the Add element dialog box).
The reason for this is that the functionality that powers the OverlayOptimizer is quite complex and making an OverlayOptimizer element which is editable and usable inside the normal WordPress pages / posts editor is quite complex.
We are looking into the possibility of adding this as a future feature, or developing some alternative functionality to make this possible, so please do stay tuned to our updates on our blog for news about this and other features in the future.