How is the star of David used in wicca?

The Jewish star of David is not used in Wicca. The Wiccan pentagram and the star of David are slightly different in appearance.

The pentagram stands on two legs and each point represents spirit, earth, air, water, and fire.

The star of David stands on one leg, and has six points in stead of five.

The six pointed star, Solomon's Seal or Hexagram (sometimes called the "Star of David") is in fact used in paganism. It has only been associated with the Jewish faith since the 17th century. Before that it was a commonly used symbol to dispel evil.