Light Thrust is now Auto-Attack, it will not appear as an action. To start AA, click a target and then you can do either:
a) Press F to enter Active-mode, click on the target again. If you're already in Active-mode just click the target again to make the cursor change.
b) Click any normal attack like Heavy Thrust, even if it is in Cool-down. The cursor will automatically change and AA will initiate.