There are three obvious routes to the cache. The shortest one from the pond at Colinton Road is also the hardest. I strongly advice against it unless the weather has been dry and your boots have grippy soles.
The entrance to the footpath from Glenlockhart Road can be hard to find but it exists. This route is probably the easiest, if not a little unexciting.
I personally recommend the trip over the hill from Craighouse Campus of Napier University. Here you have a carpark (free at weekends) and it is also the final stop for Lothian Bus no 41. Walk up to the summit at 55°55.273'N, 3°13.824'W and then down to the gap in the wall at 55°55.325'N, 3°13.983'W.
The cache is a plastic container wrapped in a green plastic bag.