It is on the underside of the cassette, 2cm from the back edge and in the middle and is down a hole about 6 to 7 mm diameter. It is usually coloured black, sometimes white, but just gently press it in with anything that will go through the hole. Also the front flap release catch is on the right hand edge, when the cassette is viewed from above, just gently press it in with anything and raise the flap to get at the tape.
Derek, May 2009