A material may have any number of textures, but the texture editor can only show one at a time; as such, in order to edit a particular one, you need to left-click on the texture's button. Right-clicking the button opens or closes the texture editor, and left-clicking selects the texture in the texture editor; double left-clicking is a shortcut for selecting a new image file for the texture.
So, if you select a BSDF from the tree, to edit the Reflectance 0 texture, you would click the right-most button in the Reflectance 0 row; when there is no texture assigned, it has a black/gray checkerboard pattern, and when there is, it has a small thumbnail of the image.
Your extreme render time with the previous grass material was due to using on-the-fly displacement on a large plane -- it doesn't matter where the material comes from, if it does that, it is going to take a very long time, and regardless whether you find materials like that on the resources site, all I can say is that I'd never personally recommend creating grass that way. If it was up to me, I'd remove any material like that from the resources site.
Also, I'll say, the new "30-grass-textures" material you got looks pretty weird, too. I can't see how it is set up in your screenshot, since I can only see the top-level Material properties, but I see it has a coating. (a little later...) Ok, I found it on the resources site, and this material is indeed terrible; it makes no sense at all, and should definitely be removed. Here is a better one
That said, please try this before downloading & using the other material; starting with the other "30-grass-textures" material:
- Right-click on Layer, choose Add BSDF.
- Click on the "basic" BSDF in the layers tree.
- Drag any one of the grass thumbnails you see along the right.
- Drag it over the new BSDF; the BSDF will be shown in the editor.
- Drag it over the Reflectance 0 texture button, and drop it.
- Right-click the "basic" BSDF and choose Remove BSDF.
- Right-click the BSDF > Reflectance 0 texture to open the texture editor
- Enable the Image Properties > Interp. checkbox.
- Set both of the texture Repeat values to 20
You will likely need to use a Repeat value other than 20, it depends on the size of the ground plane. The point of this is to show how you can quickly salvage a bad material and end up with something at least marginally useful (the texture in this particular material is not very good -- you'll see the pixels, which is why I had you enable texture interpolation in step (h) above).