You receive something only when the pool find a block, so of course if you mine for some minutes you will see 0, check again once it find a block, you should have something.
Note that mining with 300mh/s is just lost time and lost power these days
but before i was minning at 2.5 mhash with CPU (i was getting 300 with GPU) and it got like 1 block for every 30 minutes or so, so, if i mine with ATI, how much time should i wait?