You don't generate it on yourself, you get it from the SoundCloud API. You have first to authorize with your client_id, client_secret and a redirect_url then in the redirect_url (which the soundcloud server will call it should be some script on your server) you can get the token from the GET parameter "code". Then in the next step you can exchange the code for an access token.
Here is their example code in PHP:
<?php
// start.php
require_once 'Services/Soundcloud.php';
// create client object with app credentials
$client = new Services_Soundcloud('CLIENT_ID', 'CLIENT_SECRET', 'REDIRECT_URL');
<?php
// callback.php
// exchange authorization code for access token
$code = $_GET['code'];
$access_token = $client->accessToken($code);