Not 100% sure of exactly where you are, but when you entered Abismo, did you take the lower path to the right? If you did, you come on to a map with a pool of water in the top right corner. (Only giving that detail to make sure you know which map I am talking about.) On that map,
walk along the bottom part of the path. You come to a bit where the path goes up towards the pool. Where it turns up is a pillar with a pink orb on it, and over on your right, you can see an odd bit of map. The secret path is in the wall on the right of the pillar.
Follow the path through to the outside, turn the switch ON and then return to the first map where you entered. If you want to go straight on without further exploring, take the top path on the right side.
For secret paths, you can usually see where you need to get to, and the path is always (I'm pretty sure it's always) fairly direct to it.