A look at their settings for another phone gives the key information needed to set up the account. Adapting that information to the mechanism used to set up accounts on the N900:

1. Touch the clock/tray area.
2. Touch "Availability".
3. Touch "Accounts".
4. Touch "New".
5. Touch "SIP".
6. Enter yourfullphonenumber@sip3.voipvoip.com as the username.
7. Enter your password.
8. Touch "Advanced settings".
9. Touch "Use for telephone numbers", then "Done".
10. Touch "Sign in".
11. Your phone should successfully sign in. If not, voipvoip may require some additional magic setting, but I don't see anything to suggest that.

Have fun, and ask if you need any more help.