Have a look at this,
Maemo PC Connectivity
http://pc-connectivity.garage.maemo....ion/index.html

Using this you can connect your N900 to the internet through Bluetooth and USB.
Or alternatively search out the .deb files(applications) you want and transfer them over to your N900 in mass storage mode.
Once on the N900 go into filemanager and click on the .deb file you just transfered. It will open App Manager and install.

PC Connectivity is the quicker option and will connect you to internet.