sorry about the confusion - Server.MapPath is not a path - it is a method you can call in code behind.
The problem (I guess from your post) will be different.
First of all - it's not the control - it's all about security access to the network and other things like this.
Normally your site will run as "NETWORKSERVICE" - that is (if in the same AD domain) the account WebServerName$.
Take a look at the last answer here:
So the primary goal is to achieve access to the DFS folder.
By the way - here comes an extra layer of security.
You IIS Account must haver permissions to access the DFS - and the permissions on the physical path behind it.
If you are able to open (simply as response to a button click on a testpage) open a file using the DFS path you are almost done.
BUT - think about security if the server is "out in the wild" and the DFS is not part of the DMZ!!