Eleazar, it helps to pay attention to what the system is asking for It wants a command, not a URL. Besides that, you mixed things up. You start out with a filesystem path (/home/.myserver/myusername/mydomain.com/wp-content/plugins/fotobook/cron.php) but sorry - it doesn't magically turn into a URL by adding a query string to the end.
A command that starts a program is often just the name of the program or the entire path to the program file. Just like your own computer needs a browser to visit a web site URL, you need to start a program that can visit a web site URL in your cron job. You pass the actual URL to visit as an argument to the program in the command.
The command you want to run would be
wget --delete-after "http://mydomain.com/wp-content/plugins/fotobook/cron.php?secret=#######&update"
So in the command we are running the program called 'wget' and passing to it 2 arguments.
Normally wget will show status information and save the content to a file. The optional '--delete-after' argument will cause it to delete the file when it is done.
Hope this helps.