By default you cannot make a remote connection to a SQL 2008 (express) instance. To enable this you have to make the following changes:
Start the SQL 2008 Server Configuration Manager
Set at SQL Server Services the SQL Server Browser to automatic and start the service
Then go to SQL Server Network Configuration and then Protocols for Instance
By default TCP/IP and Named Pipes are disabled, enable the services you need for remote connection. You have to set these settings for every instance you want to remotely connect to.
Now you have to restart the SQL Server Services for all changed instances.