The lids on the water boiler are designed to come off, this is for cleaning inside and in future so you can replace tap washers if it's ever required.

They are held on by compression, with the outer edge of the boiler pressing against the lip on the lid of the cap.

Often in transport these are popped open, they just require pushing back, but it requires the convex side of the water boiler being pushed to fit the cap over.

The tap washer information details: 

 
To remove the lid and access the tap, first take off the carry handle from the two eyelets either side. This will enable you to remove the top cap from the boiler.

(This process is also useful should you ever wish to replace the washer or clean inside the boiler)

The top cap of the boiler will just push off and leave access to the tap and nut, these normally just need a small tighten to compress the tap washer.