In your ACP:
Languages & Phrases (Dropdown), Search in Phrases>
in the "Search for Text" enter: no_posts_in_this_forum
in "Search in ..." tick: "Phrase Text and Phrase Variable Name"
once found click Edit and enter what you would like it to say instead. I have added an example if you want to use mine:
HTML Code:
Welcome! In order to gain full access and view posts you must <a href="{2}"><strong>Register</strong></a> for an account, so <a href="{2}"><strong>sign up today!</strong></a> today!
If you have any problems with the registration process or your account login, please contact <a href="./sendmessage.php">contact support</a>.
Hope that helps!
-Roms