Hi there,
We can actually only provide domain mapping through the pointing of Nameservers, and ours are:
NS1.WORDPRESS.COM NS2.WORDPRESS.COM NS3.WORDPRESS.COM
In most cases, domain name providers can change your nameservers for you or show you how to do it. Once your nameservers are changed, you can point your mail records any way you'd like through our interface using this help article:
http://en.support.wordpress.com/domains/custom-dns/
If your domain provider cannot change your nameservers, unfortunately there is nothing much which we can do. Our system is not configured in a way that will allow for just pointing a domain at an IP address.