Download the offline
simulator
(this is a modified version of the one on the NandToTetris website). Or, if you're feeling hardcore, you can also compile
it yourself from the source code. Unzip it and then run the GUI
by running the associated script ie:
~/Downloads/nand2tetris/nand2tetris/tools/CPUEmulator.sh
You'll probably need to run the below to give the current user (you) execute permission.
chmod u+x ~/Downloads/nand2tetris/nand2tetris/tools/CPUEmulator.sh