If you don't want to take meds then your only choice is to focus and set up a strategy i.e. I used to frequently reward myself, which is highly inefficient, but it's better than not doing the work at all. Although I haven't ever been diagnosed with ADD, I've never been able to concentrate or study for extended periods, which is why I had to come up with such ideas.

Basically you work for, say, 30 mins, then you take a 30 minute break. It's a lot of wasted time in breaks, but the large reward and short duration of focus gives you more reason to try. Most meds work on the same principle and simply adjust your dopamine levels to trick you into thinking you're getting a reward for studying and/or focusing. Any reward that you enjoy will give you the same effect. That's why a lot of ADD meds are methamphetamine-based.