I had never seen this mini-assembler-based educational computer before. wikipedia.org/Little_man_computer.
I couldn't find a good online emulator, so I wrote one: Little Man Computer Emulator.
Enter the program on the left, click "Assemble", enter some inputs if your program needs them, and then step through the execution.
It's probably got some bugs since it was a quick hack, but it worked on the examples I tried it on.