Make sure the shotgun, barrel and chambers are unloaded before performing this procedure.
One of the functions of the fore-end is to push the hammer cocking rods back, in order to cock the hammers when the action in opened. Most likely the hammers are currently in the fired position which causes the hammer cocking rods (2 small pins) to protrude from the front of the receiver. With these cocking rods sticking out, installation of the fore-end can be slightly more difficult. You can re-cock the hammers while installing the fore-end as follows;
Assemble the barrel to the receiver and close the action.
Hold the shotgun so that the butt of the stock is resting on a solid surface.
Place the rear section of the fore-end at the front of the receiver as if you would be installing it, swing the front of the fore-end as close to the barrels as possible. Grasping the fore-end with both hands, pull it straight back towards the receiver, deliberately and slowly, keeping the wood close to the barrel and being careful not to slip or mar any part. When you have pulled back far enough for the hammers to fully cock, the front of the fore-end will latch in place. Open the action to insure the hammers are fully cocked.
If you are not able to accomplish this procedure please visit your selling dealer for assistance or return the shotgun to one of our service centers.