Can you manage the users using the FBA Pack User Management page? If so, then the people picker should be working as well.
If not, then you probably have an issue with your membership provider setup in your web application web.config/machine.config. The login uses the membership provider configuration from the securitytokenservice web.config - so that one is probably ok. Make sure
that the membership provider settings for your web application match that of your securitytokenservice, and that they're running under the same user accounts (or at least both user accounts have access to membership database).