It's in the network category of the application manager. After installing it on the N900, just use a Software like WinSCP on your PC to connect to it if you are not familiar with the command line. But be careful, don't change or delete arbitrary files, as that might cause your device not to boot any more. In that case, you would need to reflash. Also keep in mind that if you don't choose a secure password during installation, others might be able to connect to your phone when you're connected to a WLAN.