How to get the model into your scene ?
Go to File than choose Append. Then navigate to AlpineLimestone.blend click on it. Then click on Object. Here you can choose which Limestone you want (AlpineLimestone01 - AlpineLimestone05). This loads the model with the displacement modifier and it's material. By default it loads the 2k texture version. If you want the 4k version you have to choose Material instead of Object and choose the Material you want (for example AlpineLimestone_02_4k).
Or you can open AlpineLimestone.blend select the model you want. Hit Ctrl + C to copy the rock. Open your scene and hit Ctrl + V to import it.
Every mesh has a dedicated material so make sure you only use AlpineLimestone01 with the Material AlpineLimestone01_2k or AlpineLimestone01_4k depending one your needs.
You have 6 sliders you can use to change the material
Default value is 0.5. You can change it between 0 and 1 to shift the color of the diffuse texture.
Default value is 1. You can change the value to effect the colorfulness of the diffuse texture. 0 is black and the higher the number the more color is introduced to the diffuse texture.
Default value is 1. The slider effects the brightnes of the diffuse texture. 0 is pure black.
This effects the glossiness of the material. You can change it between -1 and 1. -1 is 100% diffuse and 1 is 100% glossy.
This effects the roughness. You can change it between -1 and 1. -1 means 100% reflection and 1 is a very rough surface without any reflection.
Default value is 1. Change this value to influence the bumpiness of your material.
Level of Detail
You can use the subdivision surface modifier to change the polycount of your object. Keep it at 0 and you have a poly count 1000-2000. Go to higher numbers and you add more detail but your polycount although increases. I recommend keeping it around 2. Higher numbers only add smaller detail. This smaller detail is although provided by the normal map, so no need to add more geometry.