Just press the knob to center the trigger on the screen then adjust it where you need it to be.
Randy
No, pressing on the Trigger level knob sets the trigger to the trigger channel's 0V baseline, which in the OP's original screenshot is _way_ below the bottom of the screen.
The whole "problem" here is that the OP is using a vertical setting of 100 mV/div and turning the Trigger Level knob over a range of 3 volts. Of course it takes some turning to do that! At 100mV/div vertical, the trigger level adjusts in increments of 2 mV, or about 40 mV per full turn of the knob, if you turn the knob slowly. There are 8 full divisions vertically on the screen -- 800 mV! So the OP is displaying only about the top 600 mV worth of a 3V signal. And at 40 mV per (slow) turn of the trigger knob ... well, do the math.
(It looks like the OP is displaying the Probe Compensation signal, in which case he needs to adjust the probe's capacitor a little bit to flatten out that trace on top...)