Here is what WordPress.com have to say on using your domain with them: http://en.support.wordpress.com/domain-mapping/map-existing-domain/
From there they are clearly saying to use their nameservers. This will because they have a number of IP address and any one of them could be the one hosting your website. Therefore they cannot reliably give you the IP address to where you would point your web traffic. Also on top of that IP address can change without any notice and then all of a sudden your website would go offline. I’d really recommend that you stick with using their nameservers.
I presume that you are using your own nameservers because you want to control other DNS and MX entries? Well if thats the case did you know WordPress.com comes with a control panel so you can setup additional DNS and MX entries?
If you still cannot use their nameservers then what you can do is set up a free account first and ping your free account domain “example.wordpress.com” and get the IP of where it is hosted. Once you have that upgrade your website to domain hosted and point web traffic to the IP you obtained. I cannot vouch if this will work but its a start.