The aqueduct is on the lower plateau of Table Mountain. It's easily accessible via several hiking routes, including Kasteelspoort and Constantia Nek. However, on a hot day Nursery Ravine and Skeleton Gorge would provide a cooler route. You can also walk from the cable car.
The aqueduct on Table Mountain was built in the late 1800's. About half way along the wall, a beautiful little waterfall falls into the channel. This is a good place to refill waterbottles and have a break. It is also a good place to see blue drip disas in December. Red disas can be seen all along the stream in February.
To reach the cache, turn off at the path (as per photo above) leading towards the rock wall. Turn right when you reach the wall, take a few paces towards the large overhanging rock and start your hunt.
Disclaimer: this cache is on top of Table Mountain and as such you need to take all the necessary safety/hiking precautions.