For successful USB-SSH connection, I have installed on my N900:

openSSH client and server
and to switch SSH ON or OFF I use:
SSH status and switcher widget

then I start on N900 bash:

ifup usb0

then on my laptop ( I use below ONE line as a bash alias !):

sudo ifconfig usb0 192.168.2.14 netmask 255.255.255.0 && ssh root@192.168.2.15

after that I am connected
when connecting USB cable - the N900 mode is PC suite mode