First thing...quick...get rid of the bioballs. If you want a fuge also, you need to remove it from under the tank, remove all the non permanant parts, and clean and dry it. Then:
1.) Get some acrylic and extend the second baffle from the right in the pics (the first is the short one by the pump, then the second one is the tall one by the foam) all the way to the bottom. The right 1/3 becomes your refugium.
2.) On the tall baffle to the right of the bioballs, at the level of the maximum line on the sticker, you will need to drill 1 inch holes all the way across, spaced about 1/2" apart. Add an acrylic piece at the bottom of that baffle, extending it to the bottom. The left section, where the bioballs are becomes your skimmer section.
3.) The center section becomes the return and your return pump needs to go there.
4.) If you have only one drain to the sump, you will need to place a "T" in the line and run part of the water to the refugium section and part of the water to the skimmer section.
5.) Add sand and algae to the fuge section, and rock rubble to the far left skinny section where the water goes in.
It sound complicated, but it's quite easy.