Texture editor UI has no scroll option / button ( very old unity version ) .
Put all your textures into folders in the Project Tab
then highlight one (you can run through them with the arrow keys) and look at the texture in the Inspector Tab. You can drag any texture onto either an object highlighted in the Inspector or in the Terrain properties of the Inspector in the Add Texture dialog you can drag any texture (must be the power of two) on to the Splat texture slot.