Bean Machine Simulation
Java has good support for OOP techniques - Using these will allow you to create a more elegant solution. Your code could be refactored with the correct use of these techniques to produce something of the form:

PegMachine sampleTest = PegMachine(500,100);
sampleTest.histogram;


This approach hides the complexities of the program and allows you to conduct tests by simply creating a PegMachine specifying the number of pegs and balls for each sample.

I noticed Aron_dc pointed out the slotIndex variable in your simulateDrop method is never used - This can be removed as it does not contribute anything to your program.

Out of curiosity I conducted a larger sample using your code to see if it produced the correct normal-distribution curve. The results were as expected; confirming your codes correctness.

Besides suggesting the use of object orientation your code seems well written and correct.