Using rescueOS is pretty straightforward, @mosen did give quite thorough rundown of the needed steps already.

Basically you need 3 things;

• flasher-3.5 tool
• rescueOS_n900_kernel_1.2.zImage
• rescueOS-1.2.img

Before you start this, your N900 must be switched OFF.

When you have downloaded those thinks to a folder, then you open command prompt and go to that folder and give this command;
flasher-3.5 -k rescueOS_n900_kernel_1.2.zImage -n rescueOS-1.2.img -l -b"rootdelay root=/dev/ram0"

After you get to the "suitable device not found..." prompt, THEN plug in the USB cord of the N900

Now what hqappens is your N900 boots up and you get to the console, there are some text helps screens.
On the command prompt you can now enter the command to remove lock code.