if you activated your DLC already via the main menu, its simply a matter of pressing B and equipping it.
The gun is pretty boss, if you use it like a rocket/mine launcher its auto-win. --- I am a qualified game developer, designer, tester and critic. I achieved all this by simply making a gamefaqs account.
It's in the single player too. However, i can't seem to upgrade it the same way i can with all of the stock weapons.
That being said, it seems moderately useful for multiplayerm, assuming you're pulling a train of aliens into your personal space. Outside of that, it feels rather useless as you'd be better off just blasting the squishy aliens with any of the available guns. --- "See you Space Cowboy..."
It doesn't need upgrading, it's overpowered enough as it is.
Let's check the stats: 70 bullets total with a mag of 10. It explodes, dealing area damage. It flies in a straight line, like a rocket. It attaches to any surface, becoming a temporary proximity mine. It detonates nearly as soon as it hits an enemy. Ammo is very plentiful for this weapon. It is powerful, being second only to a fully upgraded Battlerifle (if you headshot). It shoots as fast as a pistol. It is very accurate.
Anyway, back on topic. TC go into the game, Press B (Default control), and select the top weapon. This will let you edit the loadout, if the SHARP is not there, unlocked since the beginning, you have not activated the DLC. Try clicking the DLC option in the main menu and checking if you have the SHARP DLC. If not, contact support on the store you bought it from.