You need a fairly recent version of Unity3D, currently ECS requires at least the latest preview version of Unity 2019.1, 2019.2 should also be fine. I have tested these instructions with Unity 2019.1.0b6.
Installing the dependencies
Unity ECS is currently in preview, which means that you first need to enable the use of preview packages in Unity’s package manager. So open Unity’s package manager using Window -> Package Manager, then click on the Advanced button and enable Show preview packages.
Then install the following packages:
Installing these two packages will automatically install of the
additional dependencies (like
Mathematics) as well. After
this you may need to reimport the project so that the installed
packages are properly picked up by Unity.
Preparing the assembly definitions
After the dependencies are installed, you need to prepare assembly
definitions so you can access the Unity ECS API. To do so create
a folder for your scripts (e.g. named
Scripts), then right-click
and choose Create -> Assembly Definition. In the Assembly
Definition References section add the following references:
Now you can begin writing code with the Unity ECS API.