You most likely have the older firmware in your device which came with the devices bought some months ago. That older firmware doesn't support SSU apparently so you have to use cable to install the latest firmware.

Those who have bought the N900 later have a newer firmware which supports SSU and they will automatically get the update through App manager.

I was surprised to see that I had the update but a friend of me who bought the N900 some months before me did not have it. We started investigating what could be the reason, and finally noticed that he had different default repositories (my repository had /ssu/ in the address). Even when he changed his Nokia update repository to be same as mine, he could not get the latest update through App manager.

Apparently since I just got the device, I already had newer firmware ready installed. If you don't get the update through App manager, you need to download the full firmware and install it through cable (you'll lose all settings).