Just to explain things more clearly to a newbie (not meant as an insult btw):
There are textures, and shaders in ET. Both of these determine how a brush (a ‘box’ for example) looks and behaves.
Generally, textures determine the visual aspect of the brush. For example, they give the brush a grass or stone texture. Or in your example, an ice texture.
However, a texture alone does not determine how a brush behaves, that is up to shaders. In the shader (which is a “.shader” file, commonly found in the etmain\scripts folder) you can specify the properties of the brush you apply this shader to. For example, you can make it sound like grass, stone, or in your case make it slippery like ice. Other examples are water, being able to place landmines etc etc.
What you apply to a brush is usually the shader, not the texture. The texture used by a shader is also a property of the shader.
A shader file can hold several shaders, each with their own name and properties. If you look at Pay 'n Spray’s example, he called his shader ‘yourtexture’.
Then come a list of properties, in this case first the ‘editorimage’ which specifies the image used by the gtkradiant editor, and then the ‘surfaceparm slick’ which tells ET to make this shader slippery like ice.
Finally the ‘map’ property is set to a texture (picture) which the game uses ingame.
Now finally you might ask, why is the ice texture you applied not enough, why is it not making the brush slippery? I believe that is because you applied a texture and not a shader, and a texture alone cannot determine properties such as slipperiness. I don’t believe there are ice shaders (including slipperiness) in ET but I have found some nice ones (new textures) that do include it in the past…
So your problem was simply because there is no ice shader in standard ET, merely the texture. And in the standard ET maps, the slipperiness is caused by a second brush just above the ice with the common/slick shader applied (which is basically the ice shader without a texture).