You may not be able to equip it yet depending on what you've done so far. If you've gone up to the growth covering the forward, lower left door in the Entrance Hall it should tell you to press Q to access equipment. From there, click on the knife, then click Equip.
There's also a gun available at the first corpse you encounter. I'm not loving how the knife worked, so I expect the gun will be pretty hard to aim in combat, too. We'll see.
For the dev, any chance the aiming in combat can be improved some? Much thanks for your efforts. Also, like others, I used the key to return to the previous section of the ship, and the key is gone once you do. Can not progress in the game.