I am getting the same error with both the hello and ping tests (from the seam-gen tutorial in the reference manual). "ant test" works; TestNG eclipse plugin gives me the PropertyNotFoundException as it did for you.
Were you able to find a way to make Eclipse/TestNG work?
I am experiencing this problem as well for running the testNG test suite for the hotel booking example from SEAM 1.2.1.
where/how do you run "ant test"? in eclipse IDE?
Read the eclipse documentation.