Push RUN to run your program. A dialog box will appear when a GET is executed, and output from PRINT will appear in the right window. The simulator will stop if you Cancel a GET or push return without entering anything.
get get a number from keyboard into accumulator print print contents of accumulator load Val load accumulator with Val (Val unchanged) store M store contents of accumulator into memory location called M (accumulator unchanged) add Val add Val to contents of accumulator (Val unchanged) sub Val subtract Val from contents of accumulator (Val unchanged) goto L go to instruction labeled L ifpos L go to instruction labeled L if accumulator is >= zero ifzero L go to instruction labeled L if accumulator is zero stop stop running M Num before program runs, set this memory location (called M) to numeric value NumIf Val is a name like Sum, it refers to a labeled memory location. If Val is a number like 17, that value is used directly. So add Sum means to add the contents of the memory location named Sum to the value in the accumulator (leaving Sum's contents unchanged), while add 1 means to add 1 to the value in the accumulator.
You can also try a single-step version of the simulator, which does one instruction at a time so you can see what happens more easily.
If you want to see how the simulator works, Tools / Web Developer / Page Source in Firefox, Customize / Tools / View source in Chrome, or Developer / View Source in Safari.
There is a limit of 1000 instructions by default, to make it easier to prevent infinite loops.