Adding WordPress to subdomain

I have an add-on domain to my site and am trying to put its tables into the same wordress mysql as my other sites.

First I installed it using the Fantastico script and then I edited the wp-config file and it created the tables but now I can’t log into the admin page. The admin page has
Cannot modify header information – headers already sent by
at the top and after I entered username and password I see more of the same.

Would there have been an easier way to do this?