What are you trying to execute?
Re-run whatever was run last with Run -> Run Last Launched. Or, use the Run button in the toolbar.
To set up a runtime configuration for a Java application (or driver)
To set up a runtime configuration for a Java application which needs command-line arguments
Return to the Arguments tab to change the command-line arguments; alternatively, create multiple runtime configurations with the same main class but different command-line arguments.
To set up a runtime configuration for a JUnit test case
This will look for all test-case classes in your project, and it will run them all for you.
Eclipse will open a JUnit window. On the top, you'll see how many errors you got as well as a red or green bar. Green means that all of the tests passed; red means that there's a problem.
If there's a problem, you'll get a Failure Trace at the bottom. Each line of the failure trace indicates what code was being executed when the error was discovered. Work yourself down this list to find the offending line of code (you'll rarely have to go past the first line). You'll have to debug your computation code (usually) to figure out what went wrong.