The Texture Adjustment Menu
If you click on any of the land prims you will get a menu.

This menu lets you adjust various aspects of the texture such as scale and offsets. It also has several special modes. These will be covered later on.
Applying A Ground Texture
To apply a ground texture of your choice simply select any texture from your inventory and drag it on to any land prim.
Using Texture All
If you want to apply the same texture to all the land prims you can use the [Texture All] option from the land prim menu.

This will apply the texture on the prim to the rest of the land prims in the group.