Hello, and welcome to the wonderfull world of beermakin...
When you say its still fermenting, did you use a brewing bucket first for about a week, and then put it in the barrel?
Or did you actually start the kit off 3 days ago IN the barrel?
If you are doing the first MAIN fermentation in a pressure barrel then you will need to leave the lid loose so that the Co2 can escape. (theres LOTS of it and it will overpressurise your barrel.)
Once the main fermentation has finished, and the beer is starting to look clearish (About 7 to 10 days,no more bubbles rising), then you can add priming sugar and seal up the barrel to let the beer develope some fizz.
(although it would be better to siphon the beer into a CLEAN pressure barrel to get it off the yeast bed)
Either way if the barrel is bulging you need to let out some pressure.
Does your barrel lid have a PRV (Pressure Release Valve) as this should relieve the pressure if it gets too high.