The quick access menu is highlighted in red below. The quick access menu is also accessible from any tab you're on, so it contains all the options you might want to use at any stage in building and analyzing your simulation.
From left to right the buttons do:
Revert the changes you've just made.
Redo the change you've just undone.
Starts a single run of your simulation. When your simulation is running this will change to a pause button so you can stop it any time.
The SIMUL8 Assistant monitors your simulation building and the way you are using your simulation to provide advice to help you get it right.