The problem with packages like xampp is, that they install linux applications in uncommon places. It might be that you encounter other errors later.
The best way to install ISPConfig is to follow one of the howtos on howtoforge that prepare your linux server for ISPConfig. They where written as copy and paste howtos so that even beginners can follow them easily.
To your problem with the symlink, if i understand the xampp docs right, the symlink must be:
ln -s /opt/lampp/bin/mysql /usr/bin/mysql
But its untested because i dont have xampp installed.