Firstly, fixing a main menu 1080px to the right is not a good idea. As you've seen, it makes the site harder to use for anyone with a narrower viewport. The most usable position for a vertical menu is left.
coda-nav-1 is set to position:absolute; which means that it is taken out of the flow of the document and positioned in relation to either a parent that has position:relative; or, if no such element exists as is the case here, the body element.
If you want the menu to hang with the rest of the content of #wrapper, which is centred within body with a fixed width of 1010px, you could give its immediate parent - .coda-slider-wrapper - position:relative; and remove the left properties (there are two currently set) from #coda-nav-1. The menu will then, due to the 10px left/right margins on .coda-slider, sit 10px to the left of the left edge of the slides. If you want the left edges to align, give #coda-nav-1 margin-left:10px;
Having done the above, you'll notice that the left and right controls have vanished. This is due to their now being positioned in relation to.coda-slider-wrapper and further right than that elements fixed width. You'll need to decide whether it is best for usability for the left and right controls to be on the right or left. Again, I would go for left, as it puts them in sight for far more users. For this, remove left: 1080px; on both .coda-nav-left and .coda-nav-right. As for the menu, give them margin-left:10px if you want them to align with the slide.
Here's the result (at 50% size).