There are a few different types of towbar, but from what I can see from the picture which could do with being a bit bigger for me to see properly

it looks like there is a big plate that should go inside the boot on the floor is missing.
The big part you have with the towball on, it has 4 hole drilled into it... you will need to open your boot, pull up the carpet. You will see 4 nipples in the floor and three under the plastic rear panel which you will need to remove. They all need drilling to the size of the bolts you have (10mm iirc)
I usually fit the rear bolts first (under the rear plastic panel) to make sure the towbar body is right back in the right place before drilling 10mm holes through the floor as they sometimes dont line up and you end up with elongigated holes (hope this makes sense)
There are 2 holes further up the boot nearer to the back seats which will got through the chassis, they also need drilling.
Whilst you have the Big Plate in place draw around it and remove the sound proofing from the floor

I would remove the towbar body getting all the swarf ( the twisty bits of steel after drilling) off, put the big plate (that it looks like your missing) in place then bolt everything up

Hope that helps, I am writing this from memory so bare with me
