Well that's a little tricky. Your ABV is calculated by the difference between your OG and your FG. You are using Coopers mix, which unfortunately they don't list the gravity for, so you'll have to measure for yourself. They do say that ideally you'll end up with FG of 1.008. Let's assume your conditions are slightly less than ideal, and you'll end up with 1.012.
Now, if I plug that FG into a brewing calculator like this:
Brewheads.com - Beer Statistics I end up needing a OG of about 1.068 to hit above 6% abv. DME will yield 1 gallon of 1.040 wort per 1.1lb (500g). This means that 1 kilo of it will yield 2 gallons of 1.040 or 5 gallons of 1.016 wort. IE: For every 1 kilo of DME, your wort will go up about 16 gravity points.
So: You need to brew your coopers ale and take a gravity reading (make sure to cool the sample) Once you know the gravity of your wort, you'll know how much DME to add. Keep in mind that OG increases as you boil since water evaporates.
For example: lets say you stuck your coopes and whatever other sugars in the kettle and ended up with 6 gallons of 1.048 wort. First thing is to calculate what the gravity will be for 5gallons: 6*48 /5 = 57.6 or 1.057 OG. You need to get to 1.068, so you need 11 points. You know that one kilo will give you 16 points so 11/16 ~about 700g of DME. In reality you can just add the whole kilo and you'll be fine.
