What you're doing will return the connection string used by DevForce for this key, so I don't know why it would be incorrect. If you have multiple RdbKeys make sure you're looking at the correct key.
If this is an n-tier application be sure you retrieve the RdbKeys on the server, since you likely (and shouldn't) have the connection strings on the client.
Also, if you're using a DataSourceKeyResolver this won't return the resolved connection string for the key, but something like this will:
var key = pm.DataSourceResolver.GetDataSourceKey(name) as RdbKey;
var cnstring = key.ConnectionString;