Bleeding a Cast Iron Radiator with No Bleeder Valve
The heating system is what is called a "1" pipe heating system. It is not a hot water system therefore the radiator does not require bleeding. I can not see  the make or model number of the vent on the radiator so I can't comment on whether it the correct type. Also make sure that the inlet valve on the left side is fully open and not partially closed. Be careful when replacing the vent on that radiator  or any of the radiators when the boiler is firing, steam can come out a cause very bad burns. Is this a single family home or an apartment? If it is an apartment call the building manager and have them replace the vent. If it a single family home make sure that you turn the boiler off before working on any part of the system.