Toad is a great tool for accessing the Oracle database for development. However, there is one 'trick' regarding the configuration of Toad and the Oracle client. The Oracle client software (in my case the Oracle 11g instant client) needs to be in the Windows path. There are great instructions on how to do this here:
If the class path is not set, you will get an error when you open up Toad that the Oracle client is not found.