Sorry for the alte reply I got kinda caught up with work!
Ok, I think I understand the problem; I followed the howto but it didn't work for me as creating a file called .htaccess and .htpasswd in the munin folder did nothing.
I discovered this howto: http://www.debian-administration.org/articles/597
from Debian Admin which almost works for me now
Just the problem is that when I enter the username and password now I get an error message saying: Forbidden - you do not have permission to access munin on this server.
Can you suggest anything??