I’m trying to give you a simple demonstration of how to bleed a radiator on your own:
You should have a simple screwdriver and a dirty cloth.
This kind of radiator is common in Europe, and all you have to do to bleed excess air from it – just turn that little screw a little bit and the air will come out.
As the water (probably a dirty one) starts to come after the air, you can close that screw again. And that’s it, your radiators should be getting warmer in couple of minutes.