Please have a look in the examples folder\dc-dc there are many built examples that show carrier waveform settings. You can also have a look at any of the tutorial videos that cover a particular converter this is explained there as well.
In brief, You need a carrier waveform. The frequency of the carrier waveform will also set you switching speed. The carrier waveform should be a triangle waveform if you are generating an AC output or it should be a “sawtooth” wavefrom if you are generating a DC output. A sawtooth is important as it keeps all of the harmonic content in phase as the off transition is always at the same moment in the cycle.
In general for a DC-DC buck converter I would chose a sawtooth waveform with a peak to peak value of 1, and a frequency of whatever I want the switching to be. If I choose 100kHz then my switching will be at 100K.
The peak to peak value is important as this will be compared with the modulation waveform, a peak to peak of 1 means that a modulation value of 0.6 will give you a 60% duty cycle. A modulation of .l will give you a 10% etc.
It is often a good idea to limit the modulation waveform so that it is contained inside the envelope of the carrier waveform. You can also limit maximum and minimum duty cycles this way.
have a watch of this video, and also other videos in the tutorial library. https://powersimtech.com/support/resources/video-library/buck-inner-current-loop-design-psim/
