I am very certain it is all in the parameters of the .vmf file for your textures .vtf file. For your texture to use cubemaps from your maps you need to put this in the .vmf:

$env_cubemap

If I remember correctly when I played with textures, this did it. Good luck.

what cpt P says is absolutely true for nice equal reflections, which most weapons have (barrel etc).

however, for sake of completeness, here's how to implement "dirty" / unequal reflections (i.e. some parts dont reflect, or reflect to a lesser extent):

{
"$basetexture" "customtex/panel11"
"$envmap" "env_cubemap"
"$envmapmask" "customtex/specmask"
"$envmapcontrast" "1"
"$surfaceprop" "metal"
}

where you draw the "specmask" with black/white variations (more white =more reflective) according to your basetexture. you could also search the .gcf files for a close-matching one. it all takes a lot of time and trial/error, but the result is nice.

if you want to combine this with normalmaps (e.g. for small 3d weapon parts that you dont want to model), you have to use

{
"$basetexture" "customtex/panel12"
"$envmap" "env_cubemap"
"$bumpmap" "customtex/panel12_normal"
"$normalmapalphaenvmapmask" 1
"$envmapcontrast" "1"
"$surfaceprop" "metal"

}

instead, where you put the specmask in the alpha channel of panel12_normal (where the other 3 channels make up the normalmap)

as i said, this is just for the sake of completeness, you probably wont need it - just in case