x11vnc is the vnc server for the tablet, you don't need it if you want to control your laptop. for that, get a vnc-server like tightvnc or realvnc, install it on your laptop, set up a password, and add the tablet to the list of allowed hosts (plenty of documentation for that available on the net). then you fire up vncviewer on the tablet and connect to <your_laptops_ip>:0 (f.ex. 192.168.1.100:0) .
for the other way round (controlling / viewing your N900's desktop from your laptop), it's basically the same. you just fire up x11vnc on the tablet (the default setting is without password, and to close after one session, so you have to start x11vnc every time you want to connect, unless you set it up to start automatically, but then you also want a password, and it gets a bit more involved). then use vncviewer on your laptop to connect to <N900_ip>:0, et voilą!