I\’m not sure if you have figured this out… but I think the problem is that your container doesn\’t have enough width for both the search box and the button to fit in… hence why it drops below.
.header .form-search has a width of 400px
.header .form-search input.input-text has a width of 400px so there is no space for your button.
So, you can either
1) increase the width of the .header .form-search so its around 460px
2) you can decrease the size of the .header .form-search input.input-text so it\’s around 340px
Hope this helps!