This one is a short and easy mission. After it starts, follow Kidd. You will reach the old Mayan ruins. Climb up.
Your task here is to match the shapes of the white silhouettes with the highlighted columns. Move them appropriately to make them look like the ones in the screenshot. Then, jump down and dig out the treasure from the marked place. Then, follow Kidd again.
In the residence, you will find a grate with the Templar Armor behind it, which offers you an increased protection against the enemy attacks. To open the grate, you need to get hold of five templar keys. You already have one and you need to obtain four more. You need to complete 16 Templar Hunt missions (four per each key) to open the locks. The missions take place in Havana, Nassau, Kingston and in the Caymans. These are side missions and it is not necessary to complete them to beat the game, but the gear is good and, although what it may seem, collecting the keys is not going to be time consuming. So, it pays off to focus on that.
To complete this mission, you need to meet up with Kidd in the harbor.