The release of Debian has changed. Now "wheezy" is oldstable and without any support. "jessie" is stable now.

If you use "aptitude update" you'll get some error messages. To avoid this you need an extra file '/etc/apt/apt.conf' with the content
APT::Default-Release "jessie";
Update the cache of apt with
aptitude update
apt-key update
I've tested it with Knoppix 7.4.2 and V7.5.

Never try "aptitude upgrade" or "aptitude dist-upgrade" with Knoppix!