The 10 watch panels on the right of the assembly display can be used to watch(!) memory locations and halt execution on a write (R/W), or when a condition is met (same as branch mnemonics).
Looks like the best way to use it is to click FETCH and select one of the EPROM images (HF4WD / top one) that will wake everything up and load data to 0x8000 to 0xFFFF.
You can then assemble code, poke bytes etc then click SAVE to save the chip image to your HD. To load it back use the LOAD CHIP button. These deal with the quantity of data which was originally FETCHED, so 16k from 0x8000.
I created the emulator on a Mac running Safari, it works best (fastest) in Safari or Chrome on a PC. IE and Opera arn't great, but I think MS Edge works.