1] You don't say that you invoked
Code:
apt-get install grip
after
Code:
apt-get update
- what is the result ?

2](i) The screen shot http://www.abload.de/img/aptgetscreen21l.png is not showing a 'show-stopper' failure of apt-get update ; it is telling you that the 'unofficial' codec packages are not verified by your system.

2](ii) You can still download & install them by accepting them unverified when prompted by apt-get install <whatever-codec>

2](iii) The verification tool is installed as follows ; from a root-privileged console invoke successively :-

apt-get install debian-multimedia-keyring
gpg --keyserver hkp://wwwkeys.eu.pgp.net --recv-keys 1F41B907
gpg --armor --export 1F41B907 | apt-key add -

(I recommend that you copy & paste each complete line into the console to avoid transcription errors).

*Don't* give up if you think you see an error reported complete the instructions, then try
Code:
apt-get update
again and report as necessary.