I think this could be one of two problems.
1.By default, when you test a connection Oracle uses a dummy username and password to connect to a data source so it could be that the test tried to pass through an invalid username/password combination. To fix this look for a box that says something like change login details.
2. The error could be because your Oracle service is not set up properly i.e. you need to either copy over the tnsnames.ora and sqlnet.ora files from a valid installation or recreate the services on your machine using the client tools. Use the Oracle client tools to ensure that the service is configured properly.
I hope this helps.