If you have Photoshop CS6 or higher, you already have Adobe Camera Raw 8. What you want is to update ACR to version 8.6. You can do that automatically through the Help > Updates menu item.
If you have a lower version of Photoshop, you cannot run any version of ACR 8.x.
If you don't have Photoshop at all, then you need to buy it first.
For the next time you post, please give explicit, detailed information:
If you give complete and detailed information about your setup and the issue at hand,
such as your platform (Mac or Win),
exact versions of your OS, of Photoshop (not just "CS6", but something like CS6v.13.0.6) and of Bridge,
the type of file you were working on,
machine specs, such as total installed RAM, scratch file HDs, total available HD space, video card specs, including total VRAM installed,
what troubleshooting steps you have taken so far,
what error message(s) you receive,
if having issues opening raw files also the exact camera make and model that generated them, etc.,
someone may be able to help you (not necessarily this poster).
A screen shot could be very helpful too.
Please read this FAQ for advice on how to ask your questions correctly for quicker and better answers: