Javelin of Lightning Proficiency Bonus

I am playing D&D 5e for the first time and I have acquired the Javelin of Lightning as a Rogue. The item description for the javelin states that it is a simple weapon, but DnDBeyond is not applying any to hit or damage bonuses based on my proficiency with simple weapons. Should I have any bonuses?  

What you're almost certainly noticing is that a Javelin is not a Finesse weapon and will therefore be using your Strength modifier - not your Dexterity modifier. As a Rogue your Strength is probably pretty bad. It also wouldn't allow you to get Sneak Attack - so it's a really bad weapon choice for a Rogue.

Also - just for reference - the proficiency modifier is only ever added to the attack roll (to hit) but not the damage roll - whereas the ability modifier (str/dex) is added to both.

That makes a lot of sense. Thank you Emmber!