above ground pool on slightly sloped driveway
You won't be able to (economically) build a platform to set the pool on -- water weighs a lot! Supposing that pool is round and 20 feet diameter, when filled to 50" depth it would contain about 9800 gallons and weigh almost 82000 pounds. A truckload of sand leveled on top of the driveway is probably the only way you'd make a "platform" to support this weight.

Figure out just how out-of-level your driveway actually is by constructing a water level. You'll need a garden hose and a bucket. Using the hose, fill the bucket with water. When it is full, keeping the hose end immersed in the water at all times, disconnect the hose from the faucet and carry the bucket to the high end of the driveway. Carry the disconnected end of the hose to the low end of the driveway. Slowly move that end of the hose vertically, up and down until you see the water inside rise up to the mouth of the hose without spilling over. Finally, measure the height of the water in the bucket (above the driveway surface) and the height of the water in the mouth of the hose. The difference in these two figures is the difference in elevation across the driveway.

If the slope is only a few inches you can probably just set up the pool out of level and it'll be fine. One side of the pool just won't be as deep as the other. I'm not sure how steep is too steep, but I can tell you that I set up my own above ground pool maybe 2 inches out of level and everything worked out fine.

One other thing.. I don't mean to be a wet blanket, but consider safety. You don't want neighbor kids coming along, sneaking into the pool, and having an accident (drowning, for example). You may be taking a big risk in erecting a pool outside of a fenced yard.