Firstly you have to have a bluetooth dongle (or inbuild bluetooth, usually this is the case in laptops). When that is in order you have to make your computer and N900 a pair (this process depeds from the OS you are using). After that you just have to open your filemanager and long press the file you want to share and select Share>Send trough Bluetooth and then select your computer from the list. When you want to send a file from your computer to your N900 just right-click the file from your computer and select Send>Bluetooth device (Windows). For help with other OS'es just ask.