I have task to give in my application possibility to open document that is located on server, but I dont know at all how to implement this. Please give the main idea how can i do this?
If you are trying to open a file based on it's MIME extension and not prompt the user, you really can't do that in a Silverlight web application (for either case, MIME launching or opening a file without prompting the user). Silverlight isn't a "browser standard", and a lot of it's seemingly crippled features are done so out of the name of security, and a safer user experience. (You don't want a Silverlight app randomly reading files on your hard drive without you knowing about it and silently uploading them somewhere now, do you?)
I believe you can do whatever it is you're trying to do in Silverlight 4, but it requires elevated permissions (and consequently that it must be installed out of browser), and a little bit of COM hackery.
The elevated permissions do not give you direct access to files outside of the "My" Folders, so I don't think you are going to be able to do what you want if you want to preserve cross platform compatibility to the Mac.
However, if you don't care about that, you can do just about anything you want regarding file access by utilizing the Windows system COM objects (specifically the WScript.Shell and Scripting.FileSystemObject objects).
Here's and example. Scroll to the section of COM Interoperability.