Oh do I know exactly what you are talking about , mine was so bad I thought I skunk had gone off!
I had the opportunity shortly after I bought it to speak directly to the OLFA rep at International quilt show. Well I was stunned when he just gave me a blank look , when I asked how to get rid of the smell. He said "I have never heard of such a thing" , well not sure if his smell was off becasue the booth was filled with mats and.... REALLY smelled! After being persitant he suggested soaking the mat in a vinegar and water solution. I did that ... mimimal reduction in odor. What did work was leaving it in the garage for a few months.
I think the "Fresher " just out of the box the more intense the smell. Let it age , somewhere. Just have it lay flat, as they can warp.