index.html

<div class="hero-unit">
    <section class="container-top-left">
        <h2>Life is about FUN!</h2>
        <h3 class="slogan">Get active. Get social. Get organized.</h3>
        <p class="intro"><small>With mashup sports you will find "mashups" where everyone gets
        active and social. Get started by searching for a mashup or organizing your own mashup.</small></p>
        <span class="row"><!-- this should probably be a DIV instead of a span -->
            <span class="organize-btn"><!-- see what the css for this could look like to have a bigger button -->
                <a class="btn btn-primary btn-large" id="organize">Organize a mashup</a>
            </span>
            <span class="mashup-search">
                <p>Find a mashup:</p>
                <form class="form-search">
                    <input class="span2" id="appendedPrependedInput" type="text" placeholder="e.g. football, basketball">
                    <button class="btn" type"button"><i class="icon-search"></i></button>
                </form>
            </span>
        </span>
    </section>
</div>

<!-- here's how I'd probably do it (I haven't tested) -->

<div class="hero-unit">
    <section class="row">
        <div class="span6"><!-- a div that spans 6 of the predefined 12 columns -->
            <h2>Life is about FUN!</h2>
            <h3 class="slogan">Get active. Get social. Get organized.</h3>
            <p class="intro"><small>With mashup sports you will find "mashups" where everyone gets
            active and social. Get started by searching for a mashup or organizing your own mashup.</small></p>
            <div class="row"><!-- this defines a row inside of our "span6" in which we can add many div with spanX classes -->
                <div class="organize-btn span3"><!-- span3 => half of the remaining space -->
                    <a class="btn btn-primary btn-large" id="organize">Organize a mashup</a>
                </div>
                <div class="mashup-search span3"><!-- the other half of the remaining space -->
                    <p>Find a mashup:</p>
                    <form class="form-search">
                        <input class="span2" id="appendedPrependedInput" type="text" placeholder="e.g. football, basketball">
                        <button class="btn" type"button"><i class="icon-search"></i></button>
                    </form>
                </div>
            </span>
        </div>
    </section>
</div>

mashup.css

.organize-btn a { /* this is for all <a> tags (link tags) inside of an element that has the "organize-btn" class */
    padding: 50px; 
}