Best way to inject co2 is through pressurised co2. To put it as simply as i can, you buy or rent a gas bottle filled with co2, and slowly diffuse it into the water.
Complicated part starts now:
You need a co2 regulator to get the gas out of the co2 tank, plus a needle valve (which attaches to the regulator) so you can make the correct amount come out at a time. Regulators with needle valves can usually be bought on trademe for around $150 for an alright one. I think there are even cheaper ones there though, just single gauge ones, but even they would be fine. You then need the bottle. You can rent them from BOC gas and gear for around $10 a month, plus $30 for a fill (you need to pay for the first fill, but will probably only need it filled ever two years or so for the D size bottle (i think the smallest is D) OR you can buy a bottle from somewhere (not sure where, trademe often has them) and get it filled somewhere, but the down side to that is that you have to get it tested and certed every 5 or so years (i think)
Once you have all that stuff, you will need some tubing (cheap as chips on trademe for co2 tubing) and a diffuser or reactor of some sort. The diffuser/reactor is what puts the co2 into the water in your tank. You can get them on trademe for $20-$30 for an alright one, and a bonus for you is you could stick it in your sump (which would keep the clutter out of the tank)
The only problem you have is that by having a sump, the water gets alot of surface movement, and gets thrown around everywhere, so all the co2 you put into the water will just de-gas, and it will be pretty pointless. Having co2 on your tank would help, but not as much as it would by having it on a tank without a sump. IMO, you dont need co2, as your fish will create enough waste to keep your plant needs up, but it definately does help.
Im sure that probably confused you, because i pretty much confused myself, but i hope in some way that it helps.....