The option is not always present in sketch editing. For example, I don't see it (not even in the drop-down menu customization) in editing a trajectory of a scan.
I don't know if you've configured solidworks much, but if you haven't, you can restore the settings.
In:
Tool > Option > System Option> at the bottom of the window there is a "RESTORE" box and when you click on it you have 2 choices: Reset all options and reset this page only.