Hi IvanGamero,
in Octane, the emisdion can work only in the normal direction. In your scene, you are facing the back of the plane, and the effect doesn't work. Change the prientation of the plane to -Y, and correct the dirt and color correction node to better define the masked area. Note, temporally connect the dirt node. to the diffuse pin, and detaching the emission. In this way is more easy to find the correct values:
ciao beppe