What color is it? Are you 100% sure it's ambergris? Unless it's the inferior black grade, it shouldn't smell foul.
All ambergris needs to be ground up to a fine powder and tinctured (in 190 proof ethanol) or infused (in oil) for a minimum of six months to age and under an alchemical change. The typical dilution is 3-10% by weight. The mixture should be shaken daily, and after that you'll have an extract that is a modifier in perfumes. It's not meant as a stand-alone scent. Used at small rates in perfume, about 3%, it will help the other aromatics "marry" and improve the quality of the perfume.