You lost me on this one so I will explain both.
The messages that are posted to threads such as the messages in this post now, that you are reading are all stored in a database, it isnt a file that you can just open up as such and edit the details.
In order to access the database which by the sounds of it to me you should not be doing without first reading up on sql. You need an interface such as phpmyadmin or you can access it through a command line such as telnet. If you are paying to have your site hosted then you will need to ask them what kind of access you have to your database(s).
Now if you are talking about the messages,phrases/wording that make up the baord such as "Forums" and "Your answer" the titles, warning messages etc then they are stored in your language file which you can find in the folder [lang].