In mojoPortal we use <appSettings for our connection strings, but it sounds like your control needs to use the <connectionStrings for its connection string.
You would need to add this in Web.config after the </appSettings>
<connectionStrings>
<add name="MSSQLConnectionString" connectionString="server=yourservername;UID=yourdatabaseusername;PWD=yourdatabaseuserpassword;database=yourdatabasename"/>
</connectionStrings>
and put the same connection string value there that you have in user.config
Hope it helps,
Joe