When a program runs as a service in Windows, it cannot see "mapped" network drives. The mapped drive is something that is only available to your logged in session in Windows. The service is running under a separate unrelated account. Windows XP had a hack for this, but this was discontinued on all newer versions of Windows.
To fix this, go to your services control panel, find CrushFTP Server, and get properties on it. Set the service to specifically log on as a domain user account that has access to the areas you need. Then restart the service so it takes effect.
Next, use the left side in the user manager and paste in the UNC path to where you want to go.
Example: \\servername\share\folder\Drag and drop the folder from into the "User's Stuff" area in the user manager. CrushFTP will then add in the UNC based location for the folder, and as long as the domain account the server is running as can access that location, everything will be fine.