Try the following;
string strRptLoad = Server.MapPath("/Reports/MyReport.rpt");
Generally when you specify Server.MapPath - it gives you the root of the application directory and then follow the folders inside.
You can use Server.MapPath from within any pages - does not matter whether those pages are within sub directories in the application or not - It will give you the root of the application directory and then you would have to guide through inner folders as in the example.
Hope this helps and don't forget to mark this as a solution if it works..
Cheers