My 60Da offers an aspect-ratio option -- so I can kick it over to 16:9. The 5D II does not. But it's really no big deal. Aperture lets me do this quite easily (I just tell it I want a 16x9 crop) and it let's be place the 16x9 rectangle over the image to frame it for the crop.
The 5D II is a pro body. A pro would typically just do this in post. No big deal.
Even if you could swap the aspect ratio in the camera, you'd have to use "live view" on the LCD screen (which is NOT what you want to do when you have a DSLR) in order to know what the aspect ratio looks like so you can frame the shot. This is because the viewfinder is a physical viewfinder... not an electronic image. So you'll _always_ see the full 3:2 aspect ratio image through the viewfinder.
If you think you're going to do a LOT of shooting in 16:9 and you want a framing aid, buy a spare focusing screen. For a 5D II, you have two choices. You can either
(a) buy the Eg-D focusing screen, which looks like the original focusing screen except it has grid-lines etched onto it. The top and bottom horizontal lines are "not quite" correct for 16:9 aspect ratio, but they are _very_ close (I think they show you fractionally more than you should see). So if your subject is "comfortably" framed within those lines, you'll have no problems cropping in post.
(b) if you want precise crop marks in the viewfinder, you can buy either a spare Eg-A screen (that's the standard default screen you already have) OR you could buy an Eg-S screen (that's the high-precision focusing screen intended to make it easier to manually focus EXCEPT it's really designed for those who pretty much own all f/2.8 lenses. It needs more light.) and mark the top & bottom crop marks for an accurate 16x9 framing.
I actually PREFER shooting with crop-marks in the viewfinder and then doing the ACTUAL cropping in post RATHER than have the crop occur in the camera. Since the the sensor really is 3:2 and so is the viewfinder, why not shoot the whole frame? This then allows you to "float" the cropping so that you frame up each shot the way it looks best. If your camera pre-cropped it at the time the shot was taken then you have no flexibility to adjust it later.
I used to shoot medium format square (Hasselblad & Rollei) and this is how it works... we had the crop-marks etched on the finder screen. We'd just take the shot and set up the subject knowing whether we'd do a horizontal or vertical orientation, but then crop it after the fact. But the camera records everything as a "square" (because that's how Hasselblads & Rollies work.)
DPP came with the CD media in the box with your camera. DO NOT LOSE THAT CD!! If you lose it, then later need something from it (like the EOS Utility that you should have already installed on the camera) then you can't download it from Canon's website. They only offer the "updates" on the website... not the base software.
If you have Windows, buy a copy of Lightroom. If you have a Mac, either buy Aperture or Lightroom. They'll make managing and adjusting your images MUCH easier. Most other utilities are designed to edit photos one-at-a-time, so processing can be time-consuming. DAMs (digital asset management apps... like Aperture and Lightroom) are designed to process lots of photos in a streamlined workflow. It's MUCH faster.