You have to highlight the clip that you want to quantize. You can set it to one clip only, all clips or just a specific range by dragging across the ruler line on top.
To quantize, do a Process > Quantize then set the parameters accordingly. Set the resolution according to the note value you want to "pull" the notes to; I usually set to 1/32nd notes for the finest resolution. You can vary start times and note duration so that the notes are set to the grid pattern imposed by the resolution. The "Strength" parameter determines the percentage of pull towards the grid; 100% means it will align the notes perfectly to the grid while lesser percentages mean it will be off by the percentage amount - usually intended for a less mechanical feel. The "Swing" parameter adds something like a shuffle to the quantization. Too much swing and it sounds like the track, especially the drum track, was created after downing too many pilsens - set swing to 1 for zero effect. The "Window" setting determines quantize sensitivity meaning notes that are too far off the grid will not be quantized; a 100% sensitivity means all notes will be quantized. The "Offset" setting will move the notes from the grid by the corresponding ticks indicated.
