Ok, it sounds like we're barking up the right tree. A few more questions:
1) When you added to build path, you added the microsoft-translator-java-api-0.6.1-jar-with-dependencies.jar explicitly through "Add Jar" button, not just the /lib directory?
2) Is your project building OK?
3) Are you running your code from within Eclipse? (right click the class with your Main, and Run-As -> Java Application)
4) If running in Eclipse, is the library listed under Right-Click -> Run As -> Run Configurations -> Classpath (tab)?