Are the N900 and the computer from you are trying to make connection behind same LAN (don't know correct term but i have wifi/network switch in home where my computers are connected and they can communicate with each other, inbound connections from outside networks are blocked by service provider, some switch/adsl modems block connection between wifi and ethernet)

You have installed openssh server from repositories and gave it password in install and youa re sure that password is correctly entered to winscp(or whatever you are using to connect n900) username is root. Try to reboot n900.