Just remember the basics of what a sump is designed to do for us;
A sump is an area external to our tanks (and usually below), that allows you space to place filtration and heating.
If your case, the added light will also give you a place to have as a Refuguim in conjuction with your sump.
Typically, water comes into the top of the sump on one side or the other. The layout of left to right, or right to left... is entirely up to what will work out best for your case. I have typically run mine left to right, since that is the way I tend to think.
On your sump, it looked like there is a bulkhead fitting on what will be your "Input" section. From that bulkhead up to the overflow box, you will want to add plumbing (Spaflex flexable piping, or hard PVC piping both will work just fine) connecting the overflow box down to your sump. Sorry, I didn't find a picture of this yet, but I'm sure people have nice pictures of their setup, and will join in here showing you this.
Okay... so we have a way for water to get down to your sump.
The center section, is where you would add your heater... so it no longer needs to dangle into your main display tank. This helps keep your display more astetically (-5pts speelin) pleasing, yet keeps your water the temp you want it at.
Because you are planning on using that center section as a Refuge... you can probably also put your heater into the 1st section just as well. If it fits, it will work there just fine.
For now... lets just leave the center section empty... with the exception of perhaps sitting the light over it.
Now... the last section, is where your return pump sits (it is a submersible pump)... as well as your skimmer and its pump.
The best pictures I could find quickly here trying to help show you what a sump looks like... I have borrowed from Nikki (NaH2O) and Skimerwhisperer, from this thread;
http://www.reeffrontiers.com/forums/showthread.php?t=10264&highlight=sump
On Nikki's sump... you'll see her Skimmer in the far Right chamber... rock (Fuge?) in the center... and her return pump is in the far Left section.
On Skimerwhisperer's picture... you will see his skimmer and pump in the left side, and the return pump in the right side.
Hopefully, these pictures can help a little with showing you how various sumps can be setup.