This page seems to have the variables you need:
http://www.mailpress.org/wiki/index.php?title=Manual:Api:Mail
If you use $args->toname in your email code somewhere, that should display their name.
Please note that I just skimmed through the site and found this, so I can’t guarantee it works since I don’t know what your current code looks like nor have I used the plugin, but it looks like this is what you’re looking for.