As a matter of fact, you do have a throttle. Based on what the instructions say, the throttle is the left stick, and the horozontal control is the right. Based on the fact that it is a 2-channel plane, someone might need to correct me. With the two channel, the way that you level off is by reducing the throttle. You reduce it enough so that you do not climb and dive. You will most likely have to constantly adjust it, along with the steering control.

Here is a step-by-step list of what you should do when you fly.
1. launch the plane by hand. Point it level to the ground and have full throttle.
2. After launch, adjust the throttle (ever so slightly) in order to have a constant climb. Make sure that it is not too steep though. That is probably the reason why the plane kept diving. You probably stalled it. Just make sure you have an aerage rate of climb.
3. When the plane is at a decent altitude, gently begin turning the plane. You dont want to hold the stick all the way to one side. Move it just enough to keep the plane at a nice turning radius. It doesn't have to be a very tight one. Just take it easy.
4. Now, you will eventually get to a point where the plane is facing you. This is when you will experience a thing called control reversal. This is when you move the steering control to the left and the plane looks like it is going right. It takes a while to get used to, but one way to help yourself is to move your body so that you are facing the direction that the plane is flying. This helps orient yourself with the controls.
5. After you have done a few circles and feel comforatable, get the plane to a good height above the ground and lower the throttle. In doing this, you will get an idea how the plane will handle during landing. Do this a couple of times before you try to land.
6. Make sure that you land well before the battery runs out. This will give you more chances to try if you happen to miss the landing.
7. The landing is the most fun thing about flying. Start by flying with the wind to the side of your runway. Then make a crosswind turn and lower your throttle a little. Then turn into the wind and lower your throttle some more. If needed, you can even cut the throttle. Next, let the plane glide to the runway (make sure the plane has its nose slightly down but not a lot). Since the plane has no elevator, I don't know how to flare the plane, or if it even needs it. I am guessing the craft will just touch down nicely. Someone help me with this part.
8. Celebrate because you just made your first successful RC flight.

If you would happen to crash, dont get upset. I have crashed many times, but I am still out here flying. It will take a while beore you can be confident in your abilities.

Another tip is to download some videos of RC planes flying. This will give you an idea on how they will react in the air. Make sure that they are slow flight trainers though. Don't get your flying tips from a jet that goes 100mph.

Good luck and welcome to the best hobby in the world.