You’re on the right track.
The first piece of code is this:
.main-navigation {
-moz-border-radius: 15px;
-webkit-border-radius: 15px;
border-radius: 15px;
}
That would be it, except that now we have to round only the left side of the first menu item.
.main-navigation .menu li:first-child a {
-webkit-border-top-left-radius: 15px;
-webkit-border-bottom-left-radius: 15px;
-moz-border-radius-topleft: 15px;
-moz-border-radius-bottomleft: 15px;
border-top-left-radius: 15px;
border-bottom-left-radius: 15px;
}
That should do it.
Let me know π