There should have been a Details button for you to click on, as having the details would be very helpful.
There are three main layers of things to get through here:
1.) Did you download and install the Tableau 8.0 Oracle Drivers? http://www.tableausoftware.com/drivers
2.) Do you already have a working TNSnames.ora setup on the same machine as Desktop? e.g. do you have TOAD or other Oracle-connecting products up and running? http://kb.tableausoftware.com/articles/knowledgebase/oracle-connection-errors
3.) Are you absolutely, positively sure you used valid credentials? Pay particular attention to the Service ID.