Jesus, someone's in a bad mood. Even though I gave you everything you needed to know, let me spell this out for you reeeeal slowly.

1) Create a brush that will be your button (this is with the brush tool -- it looks like a cube on the left side).
2) Tie it to an entity with CTRL-T (that's called a hotkey).
3) Select the func_button option (it's in the little thing below Class: -- it's called a dropdown box). We're just going to leave this for now. Don't touch it. I mean it.

4) Click the entity tool (it looks like a light bulb).
5) In the 3d view click the entity somewhere, or, alternatively, in the 2d view, click a point, arrange it according to your specification in the 3 2d views, and hit the Enter key (it has a little down left arrow on it usually).
6) Pat yourself on the back, you made an entity!
7) Hit Alt-Enter to bring up the properties (yes, another hotkey).
8) Change the class to Light. That means it's a light (one that doesn't move and goes in all directions -- like a light_dynamic or light_spot).
9) Give it a unique name. This would be by clicking the Name parameter and typing something in (and unique means nothing else has the same name).
10) Mess around with the brightness and color if you so desire. In the Flags tab you can set it to Initially Dark if you want (by checking the box there).

11) Go back to the button (the one you made in step 1), bring up the properties, and go to the Output tab. This is how you get the entity to o-u-t-p-u-t something to another entity (AKA change its settings).
12) Hit Add.
13) Hit the little down arrow next to the "My output named" textbox.
14) Select OnPlayerUse (makes sense, right?....right?)
15) In the "Target entities named" box, type the name of the light you made earlier. Alternatively, you can hit the eyedropper and left click on your light (make sure the name that comes out matches that of the light).
16) In the "Via this input" box, you have a multitude of options. "Turn off" means this button will turn off the light, "Turn on" means this button will turn on the light, and "Toggle" means it will turn the light off if it's on, or on if it's initially off.

There, your majesty, is your step-by-step tutorial. (and yes, THIS is what it looks like when I try to talk condescendingly)

In the future throwing complaints and accusations at people who are honestly trying to help usually doesn't work, because the main reason people do this is for the gratitude of the person they helped. Now, even IF you do all this and even IF it works for you I won't expect positive feedback not just because of how I conveyed my response, but because of your negative response to my initial comment.