Run the following from command line.
netstat -n -a |findstr "1433"
If you get something like the following, TCP/IP is configured for the sql instance. If you don't get anything, you need to configure the SQL Instance for TCP/IP. You can follow the instructions on MS's site for this: http://msdn.microsoft.com/en-us/library/ms190608.aspx
TCP 0.0.0.0:1433 0.0.0.0:0 LISTENING
If you don't know what the name of your SQL Instance is, follow these instructions:
Open Microsoft SQL Server Configuration Manager
Select the "SQL Server 200x Services" item on the left.
Your SQL Server instance will be listed on the right along with a few other services.
If the name of the SQL service is "SQL Server (MSSQLSERVER)" you are running a "default" instance so you do not need to specify the instance name in your connection string. If the name of the SQL service is anything other than a default instance you must use the name specified in the parenthesis in your connection string. Example, if the Service name is "SQL Server (SQLEXPRESS)":
<add key="MSSQLConnectionString" value="server=.\SQLEXPRESS;UID=mojouser;PWD=P@ssword1!;database=mojo" />
These are all just troubleshooting steps to help you gather more information. If you follow them, you will more than like run into the problem and be able to fix it.
Shameless plug: I will be more than happy to login to your machine and figure out what is wrong with your configuration. If you are interested in details and pricing, email me at joe (at) i7media (dot) net.