Coopers recommends using their Brewing Sugar pack, which is 1kg of dextrose. So 1kg or approximately 2.25lbs is about right.
However, I would recommend using a couple of pounds of Extra Light DME instead of dextrose (Coopers brewing sugar) or sucrose (table sugar). My first 4 brews were Coopers kits + dextrose, and although they were drinkable, they weren't very good ... every one of them came out very cidery, even after 3 weeks in primary and 3 weeks in bottles as opposed to the 1-2 method suggested by the kit instructions.
Some people disagree, and say that the Coopers kits are made to have plain sugar added ... this school of thought says that using DME instead of sucrose/dextrose would make the beer too malty.
Like many things in brewing it appears to be a matter of personal preference.
So it's up to you what you want to do (of course), but that's just my $.02 based on my experience. If I had it to do over again, I would have tried DME instead of sugar.