Your class file (.class, NOT the .java) needs to be in a folder named testPackage, so it's relative path is testPackage/TestClass.class
Assuming you have the folder testPackage at C:, so the full path for TestClass.class is
C:/testPackage/TestClass.class
from the console you would type
java -classpath C:/ testPackage.TestClass.
Hope this helps!
Rob