Cannot find Underground Jungle Temple

Solution 1:

What you're talking about is the Jungle Temple, or Lihzahrd. It is definitely in your world, so long as your version of Terraria is 1.2. I found mine by accident pre-hardmode. It actually took me longer to find the Beehive then it did for the temple. And my temple was actually well above the lava level of the area.

For reference, the temple is going to be around 1/6th the full size of the jungle. You'll find it eventually. It is, indeed, a pyramid shape with the top of it containing the entrance and the bottom containing the Altar.

Your best bet would be to continue exploring the jungle pre-hardmode. Keep to the larger areas and don't start digging sideways until you can't find a way through anymore. Bombs also help with clearing the area and won't damage the temple if you find it, since temple blocks can only be harvested by the best hardmode picks.

Solution 2:

Spelunker Potions are a decent way to help find the Temple. They make pots and chests glow, and the pots and chests in the Temple have a different look than the ones in the Jungle.

However, if you are just tired of looking and willing to go outside in-game methods, you might benefit from the MoreTerra utility. This creates a visual map from a world file, allowing you to easily find points of interest.