In Infoview, right click on the report in question and select Properties. There you'll see the file name, something like:
File Name:frs://Input/a_145/066/051/3359377/x-guid[av.v4l30pi1iov1mj3kjxem].rpt
That is the physical file location, but you have to know where the Input FRS directory is. It's probably on the BOE server, take a look in the Business Objects Enterprise directory and see if you find a directory titled Filestore. If so, you'll see subdirectories Input and Output - then follow the above path to get to the report you want. If you can't find it post back.
However.. if you just want to modify the report, you can do so from with Crystal. Select Open, and in the left pane of the dialog you can scroll down to the Enterprise icon. Click it and you'll be prompted to log on, then you can navigate to the report you want. Make your changes, save, and the report will be updated in BOE (usually - post back if the changes aren't reflected).