use the level to get the table close to level. then i always use the ball to fine tune it... after all it's the ball you want to roll right in the end anyways. When leveling the table make sure the table surface is clean and it is always preferable to have a new or like new ball, or at least a ball that you are confident will roll straight.
What i do is put the ball directly under the goalie 2 bar at one side of the table. then push/pull(depending on where you place the ball) and see if the ball travels directly under the rod. Keep in mind you have to pull completely straight and consistent, otherwise you will make the ball roll forward or backward and it will look like the table isn't level. and also pull it fairly slow so the ball barely makes it to the other wall. If you know your pulling straight but the ball moves forward or backwards adjust the table leveler legs accordingly until the ball rolls completely straight.
next move the ball to the opposite side of the table to the other goalie 2 rod and repeat. if the surface is warped it will be impossible to get both sides perfect. if the surface is flat the second goalie should need little to no adjustment.
Now that is how you adjust it on the length of the table but you also need to check side to side... it is the same procedure but instead of keeping the ball under a rod you need to roll it down the length of the play field to see if it wanders. the easiest way to accomplish this is to place the ball on the outside goalie line underneath the goalie 2 rod. push the ball down the length of the table using the guy and see if it rolls onto the opposite goalie line and if you hit it hard enough it should come back and hit the same guy that hit it to begin with. if the ball wanders adjust the legs accordingly.
Now i hope that all makes sense, if not let me know and i can make you a quick video and put it on YouTube showing you exactly what you need to do. and hopefully that will be better if you can actually see it done.