I used to use a pressure barrel several years ago, and I would think it was similar to the one you have.
To fill it, remove the top (there won't be any pressure in an empty barrel), and clean and sanitize everything. Prepare your priming sugar by boiling it in a small amount of water for about 15 minutes, cool it so that it won't damage the plastic when you add it to the barrel, and pour into the barrel.
Then (using a sanitized racking cane) siphon your beer on top of the sugar solution, leaving the trub behind in the fermenter.
Screw on the top, and leave it in a warm place (20 C plus) for at least 3 weeks. During this time, the yeasts will ferment the priming sugar and generate CO2 which will cause pressure to build up.
The attachment on the top should be for a CO2 cartridge which can be used for dispensing the beer, and preventing it from going flat. I used to give it a quick shot of CO2 after every 4 or 5 pints which worked well for English Bitters.
Hope this helps.
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