You can test the mysql connectin like this: Open the file /usr/local/ispconfig/server/lib/config.inc.php. Inside the file, you will find login details to the local database and to the remote (master) database. Use the login details for the master database to execute this command on the shell of the slave to test the connection:
mysql -h masterhostname -u username -p ispconfigdatabasename
The command will query you the for the password.