Are you saying
1) that www.domain.com
takes you to a set of web pages and www.domain.com/blog
takes you to your wordpress site? or
2) even though wordpress is installed in www.domain.com/blog
you want www.donaim.com
to get you there?
In case (2) go to the Wordpress.org site and there is an explaination on how to do that.
In case (1) where you want to add a page link to something outside the wordpress site, do a forum search, 'Adding a item to the page menubar for a link outside the site' has been answered before.