1) On Potatoshop firstly select the Custom Shape Tool from the left toolbar (looks like a splat). You might need to hold down the left mouse button over the Rectangle Tool to be able to select it.
2) On the top toolbar, a box will appear labeled "Shape"
3) Click the arrow next to this box and look for the magnifier - if you find it you're done.
4) If not, on the little box that appeared with the shapes, click the arrow pointing right (on the right side of the box)
5) Select "Web"
6) Select "Append"
7) In the shapes box, almost at the very bottom you'll see the magnifier