Now move the entire rung for the solenoid logic so it is above the "Box Filled" logic. Then you will see that it is back to spilling material (if I:1/4 is in the Box Level rung but not in the solenoid rung). So what really fixed it?
Normally an accurate level indicator should be used to control the solenoid that causes the level.
If you successfully complete the entire set of lessons and simulations on Logixpro, you will be ready to program with Logix 500. The problem solving, troubleshooting, and knowledge of how a PLC works and interprets your program will be extremely valuable. I would recommend looking at other brands of PLC's for trainers. I know Mitsubishi has a trainer/simulator for the FX series for example. Learning multiple platforms will help when looking for a PLC related job.