-.-''' mora.... every device, from iphone to any nokia from the First World War, has some system directories.
Install FILEBOX and give a look to the structure of the directories..

When you plug you n900 to the pc in mass memory mode, the file are stored in /home/user/MyDocs , that is even the directory "N900" in the default file manager on the device.

So if you have put the deb file in "N900" (this name can change if you change the name of the device in the bluetooth settings...) you will have to type in the terminal :

sudo gainroot
dpkg -i /home/user/MyDocs/maemodder_0.8-3_all.deb

If you don't understand now... look for a bridge and use it xD