1. Go to your admin panel
2. go to modules > forum widget management and then create one
3. fill with this :
widget name : Guest message by nimbuzzmasters
use a table type : no
widget title : leave it empty
widget source use the code below

Code:


        <div class="unregisterbox">
        <div class="imp">!</div><strong>Welcome to (yourforumname)</strong>
        Join us now to access all of our features. Once you join us, you can make any topic to talk in this forum, given the reputation of a fellow member, design your profile, and much more. If you already have an account, <a href="/login">Login here</a>. If don't create one, <a href="/register">here</a>.
        </div>
        <style>
      .unregisterbox {
    position: fixed;
    left: 20px;
    z-index: 30000;
    background: none repeat scroll 0% 0% rgb(207, 55, 55);
    color: white;
    text-shadow: 0px -1px 0px rgba(0, 0, 0, 0.3);
    border: 1px solid rgb(119, 27, 27);
    box-shadow: 0px 1px 4px rgba(0, 0, 0, 0.3), 0px 1px 0px rgb(246, 108, 108) inset;
    border-radius: 3px 3px 3px 3px;
    bottom: 20px;
    text-align: justify;
    padding: 10px;
    text-decoration: none;
    font-size: 12px;
    font-family: Lucida sans;
    line-height: 20px;
    height: auto;
    width: 400px;
}
      .imp {
    display: block;
    float: left;
    font-size: 150px;
    text-shadow: 0px 0px 10px grey;
    height: 116px;
    line-height: 0.8;
    color: white;
}
        #lapis {
            background-color: black;
            opacity: 0.5;
            width: 100%;
            height: 100%;
            position: fixed;
            z-index: 30000;
            top: 0px;
            left: 0px;
        }
        </style>

and then, save it
4. if done go to your forum widget management again then set permission just for guest
5. uncheck all except guest, then save it

now its done