How to add leak sealer?

I know I have to replace it but moved into a house not even 6 months ago and boiler started leaking recenlt. So doing this as a temporary fix. I will buy a leak sealer from home depot. Where do I put it in? I looked around the web. Picture of my boiler
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I have this boiler http://www.peerlessboilers.com/Products/ResidentialBoilers/SeriesWVDV/tabid/117/Default.aspx#dnn_relateddocuments
The instruction says pour it through safely bulb .Any help is appreciated. Sorry for the noob question.