Conceptually the ATS is quite different from a 737 type of autothrottle. Let me try and explain it simply:
The ATS will only work if there is a valid thrust rating. Click the PERF button on the CDU, and make sure it shows <ACT> next to a rating.
You can use the TOGA buttons at any point, even on stand with the engines shut down, to arm the flight directors and ARM the autothrust system. It will then show "N1 TO" on the ATS matrix display. If the flight directors are already showing, you must turn them off and then press TOGA to arm the ATS.
For takeoff, just push the power levers forward or press the "ATS" button on the FCP. This will then engage the ATS to give you takeoff thrust.
If at any point you have FAIL on the matrix display, you must use the ATS Disconnect, (look for at_disc in the bindings) to clear the matrix display before reengaging the ATS.
Hope that makes enough sense to get you going.