First: try another Wifi device first as another poster suggested.

Never forget to make a backup on your N900 before
installing a new software.

There are always different ways to get software onto your
N900:
connect N900 via USB cable to your desktop or laptop machine and copy the file to the partition that you like
or
set up bluetooth and transfer the file via bluetooth.

If you used USB cable do a disconnect of the hardware in the
tray if using Windows or unmount the device if using Linux,
then unplug the USB cable.

If you used bluetooth switch off bluetooth on the N900.

Then use the file manager and go to the place where you saved the software, selecting the *.deb should try to install it.