You need to start in the path where class files are and add the needed libraries.
For example:
<ClasspathInYourProject>java -cp .;../lib/protobuf-java-2.4.1.jar;../lib/protobuf-socket-rpc -2.0.jar;../lib/TTmanServerAPI.jar com.testingtech.tworkbench.ttman.server.example.client.TTmanExampleClient
Alternatively, and more easily, you can export it as jar file over File > Export > Java > Runnable JAR file.
You can run the created Runnable JAR file using the following command line: java -jar <jarFileName.jar>