There are two methods the body uses. First there's just numbers and random chance. In the clip you're mentioning, it could be that the white blood cell just happened to be moving that way. There are likely other WBC in the slide that aren't moving towards bacteria.

The other method is chemical messages. Bacteria produce waste wherever they are. WBC are drawn to the waste and to pieces of bacteria. The cells in your body also make messenger proteins when they're attacked/damaged by bacteria. The WBC follow these messenger proteins from low concentrations to high concentrations.