I want an entity to glow when for example hit by something. The routine I am trying to use is:

MATERIAL* glow_on =
{
emissive_blue = 100;
emissive_green = 100;
emissive_red = 255;
power = 255;
}
MATERIAL* glow_off =
{
emissive_blue = 0;
emissive_green = 0;
emissive_red = 0;
power = 0;
}

function main()
{
...
entity.material=glow_on;
...
}

However nothing happens to the entity, it just stays the same. Can someone tell me how to make an entity glow.

What is "entity" and how do you initialize it? And for gods sake, please use code tags!!11

Then you must set the entity's material to glow_on, when it is hit by something (e.g. via an event) and when it is not hit, then you use by default glow_off as entity material. Simple.

HeelX he's talking about that the material isn't doing anything...

I used entity just to describe, it is an object created with med which is displayed with ent_create. What however I want to mean is that if I use:

entity.ambient=100;

The entity lights up, but I want only the red component to light up so I used the material above, however it does not work.

With your material definition it could barely be visible. The overall lighting of an entity is a bit more complicated. You can see the full documentation here: http://www.conitec.net/beta/lighting.htm

For a start try to set all other material parameters at 0 in order to remove the effects of lights in the environment.