You have to change the file ./include/staff/login.tpl.php
In this file you will see:
<table border=0 align="center">
<tr><td width=100px align="right"><b>Username</b>:</td><td><input type="text" name="username" id="name" value="" /></td></tr>
<tr><td align="right"><b>Password</b>:</td><td><input type="password" name="passwd" id="pass" /></td></tr>
<tr><td> </td><td> <input class="submit" type="submit" name="submit" value="Login" /></td></tr>z
</table>
Change it something like:
<table border=0 align="center">
<tr><td width=100px align="right"><b>Username</b>:</td><td><input type="text" name="username" id="name" value="" /></td></tr>
<tr><td align="right"><b>Password</b>:</td><td><input type="password" name="passwd" id="pass" /></td></tr>
<tr><td> </td><td> <input class="submit" type="submit" name="submit" value="Login" /></td></tr>
<tr><td colspan="2" align="center"> Please login with you OWN username and password!</td></tr>
</table>