Getting custom membership provider to work with SQL Server 2008

Hey all, I'm trying to work on writing a Custom Membership Provider so of course the first thing I try to do is setup a connection with Sql Server 2008 which i'm running locally.  I am having the most difficult time getting my SQL Server login to work within the web-application.  Here is the error I am getting:
 
Login failed for user 'syntheticUser'.
which is occuring on the following line of code:
SqlConnection conn = new SqlConnection(_connectionString);
conn.Open();
It is a SqlException (class 14).
 
Here is what I have done so far:
1. In Sql Server I have created a database called PhotoSyntheticsDB which has a table Users.
2. I have created a Login named syntheticUser with a password: testPass (for our example)
3. I have set syntheticUser's User Mappings to PhotoSyntyntheticsDB with datareader, datawriter, dbowner, and of course ...