You may want to search the forum for how to revive a very dead battery. The new one you got may have been discharged, since it is unlikely that any (real) Nokia battery is all that new these days, even if unused. It can be a pain to revive a dead battery that will no longer charge when you plug it in with the charger. But usually you get some sign of life (charging starts when you plug in but then stops). Then, you can probably use the Flasher to charge the battery. Also, be sure to do a long-press of the power button, at least 10 seconds, otherwise the phone is not completely off and Flasher won't recognize it when you connect it to your computer.

For example, see this thread:
http://talk.maemo.org/showthread.php?t=87709