From the manual:
"When first starting Rockbox Utility run â€œAutodetectâ€, found in the configuration dialog (File â†’ Configure). Autodetection can detect most player types. If autodetection fails or is unable to detect the mountpoint, make sure to enter the correct values. The mountpoint indicates the location of the player in your filesystem. On Windows, this is the drive letter the player gets assigned, on other systems this is a path in the filesystem."
The manual is an invaluable source if information...