BBappfx is configured to use Integrated Security.  Modify the ConnectionString parameter in C:\Program Files\Blackbaud\bbappfx\vroot\web.config to reflect the following:

<add name="BBInfinity" connectionString="Server=<database server>;database=<database>;user id=<database user>;pwd=<database user password> " />

The connection string should point to the BBNC Database, and the user id and pwd fields should reflect the sql account that has ownership of the NetCommunity database.