The executioner is a different build for the assassin, which grants different powers and a different overall feel The assassin is good at dealing damage and lays down "shrouds" on their targets, which, when activated, deal more damage. An executioner specializes in poisons (for attacking during role play, out of combat situations, or in combat), and uses several different weapons. The reason you've been given your powers is because the executioner is an Essentials build; the Essentials line is designed for complete newcomers to D&D, and does a lot of the work for you. If you have experience with 3.5, this could be part of the confusion.
Try going back into the builder and see if you can switch to just Assassin. I don't use the online one myself, so I can't tel you how you could do that, but the option should be there somewhere. Then you can pick your at will powers, and you're only using one weapon so you don't need to continually switch around. I'd recommend using the assassin on it's own, as I think it's a bit easier. Also, with the executioner, your expertise feats won't work for all your weapons, so you'll either need to spend more feats to cover everything, or you have one great weapon and a few not so great ones. (For example, Flail Expertise gives you a +1 to hit with flails like the bola, but not for anything else...like the dagger or blowgun, which your other powers use).
You might also want to consider a different class. If you want to be stealthy and hit hard, maybe try the Rogue. Their Sneak Attack feature is strong, and coupled with the feat Nimble Blade, you can be hitting an awful lot. One of the build features of the rouge lets you gain concealment with a stealth check after a move action--I think it's called Cunning Sneak.
There's also the warlock. They fight at range, but have a great feature called Shadow Walk; whenever you move more than three squares, you automatically gain concealment. I'm playing one currently, and am having a lot of fun.
At any rate, Cnyprios has the right of it: if you're starting at level 5, you should also be starting with some magic items, as well as more than enough gold to buy a couple more. If your DM is starting you of with 100 god (which is for first level characters), you can't afford any of the stuff that you're supposed to have by level 5. The system is designed with a certain scaling of damage/ac/etc. If you don't have magic, you'll be very easy to hit, and it'll be very hard for you to hit monsters.
For example, the average to hit bonus of a Brute monster at level 5 is around +8. When they roll a d20, they add 8 to the result and balance that against your AC. Which means to hit you, they only need to roll a 9 or higher to hit you--they'll hit more often than not. Likewise, their AC is going to be around 17; if you're not using a magic item, your attack bonus should be about 7, meaning you need to roll a 10 or higher to hit; you'll hit them a little more than half the time. This all may sound balanced, but what it amounts to is that you're going to have some difficult encounters. The way the system works, you should be hitting more often than not, so no encounter should have only a 50% chance of winning; you should be winning most of the time.
But, if your DM doesn't want to use magic items, there's the Inherent Bonuses rule--which basically does the same thing. It gives you bonuses depending on your level so your character can keep up with the math. Otherwise, you're going to be failing encounters and dying a lot.