Loop points can't be set within FL Studio itself. They are part of the sample data, they can be set using Edison, Slicex, or an external wave editor.
Edison and Slicex are included in FL Studio Producer Edition & Signature Bundle and can adjust loop points in sample files.
When you play looped wave samples in FL Studio and you hear pops and clicks this indicates that the loop points of the sample are not well chosen. Well chosen loop points match end offsets (amplitudes) of the sample points and also match end slopes of the waveform as accurately as possible.
Further, if the sample volume varies over time, you should set loop points so that level is equal or close to equal in the start and end loop points.
Finally, unless you're making a sample to be used as an oscillator shape, you should avoid creating very short loops (less than ~50ms) because they may sound off-pitch, not matching the rest of the sample.
Image-Line Support Team