please run alternatives first and select what version of mydns you want.

su -c 'alternatives --config MyDNS'
Password:

There are 2 programs which provide 'MyDNS'.

  Selection    Command
  -----------------------------------------------
  *+ 1           /usr/sbin/mydns-mysql
     2           /usr/sbin/mydns-pgsql

Enter to keep the current selection[+], or type selection number:

if you want mydns with mysql support please answer 1, otherwise answer 2