Press U on the N900 and hold it down while you insert the USB cable into the computer. KEEP HOLDING "U" until the flasher sees the phone. This means you should have already started the flasher app with flasher -R xxx.bin -F -r (not quite sure on the switches, haven't used it in a few days).

After your N900 is flashed and up and running... stay away from the extras devel repos. Even extras testing, for that matter. Sure, we need people who can test apps, but (and don't take this the wrong way), you're not one of those people. If you can't figure out how to reflash your device, don't even know about roofs and you think all the apps go onto the 2GB drive and if you panic like a little school girl if your N900 starts acting up, then the above mentioned repos are not for you.