I dont know the point of entry from the TestNG eclipse plugin.
But you can try setting up breakpoints in org.testng.TestNG class in perhaps the below methods
configure()
run()
Alternatively just to narrow down the problem, you can try creating a main() method which instantiates the TestNG object and check if the run() works fine. That way we would get to know where the problem is.
public static void main(String[] args) {
TestNG testng = new TestNG();
testng.setTestSuites(Collections.singletonList("src/test/resources/mysuite.xml"));
testng.setVerbose(2);
testng.run();
}