Looks like you got most of it working already. To add a Bolder font, take a look at line 1293 in your CSS and change the font-weight from 300 to either 400, 600, or 700+ –
.genesis-nav-menu a {
color: #fff;
display: block;
font-size: 13px;
font-weight: 300;
letter-spacing: 2px;
padding: 30px 15px;
text-transform: uppercase;
}