Once you have imported the node group, you can access it. You do this by clicking on “Add” in the geometry nodes tree and then selecting the imported group under “Group”.

1. Save the downloaded file on your pc, where you can find it later.
2. Open the blender file in which you would like to import the node group.
3. Go to File --> Append --> Multi Index Selector for Geo Nodes.blend --> NodeTree
4. Add "Multi Index Selector"

You now have the group node in your blend file. However you won't see anything different. 
If you don't know how to set up the group in a geo nodes system read the FAQ "How to use the node group".