if you're just trying to set up a yahoo email account on the n900 then you need to work with messaging.nokia.com - go there and set up an account, enter your yahoo mail info, and then activate a nokia messaging account on your n900.

It syncs great and instantly. haven't been able to get the subfolders to work, but i think i just didn't try hard enough. be careful when you delete emails on your n900 - they will also be gone from your yahoo email, for good! personal knowledge, don't ask...

good luck,