Requires installation of untrusted packages

I’m brand new to Linux and I’m having trouble installing various plugins etc.
I get the following error when I try to install VLC, Skype or plugins for the Music Player:

Requires installation of untrusted packages
The action would require the installation of packages from not authenticated sources.

gstreamer0.10-ffmpeg gstreamer0.10-fluendo-mp3 gstreamer0.10-plugins-ugly liba52-0.7.4 libdvdnav4 libdvdread4 libid3tag0 libmad0 libmpeg2-4 liboil0.3 libopencore-amrnb0 libopencore-amrwb0 libpostproc51 libsidplay1 libtwolame0 (Music Player Plugins)

and

ia32-libs lib32asound2 lib32bz2-1.0 lib32gcc1 lib32ncurses5 lib32stdc++6 lib32v4l-0 lib32z1 libasound2 libc-bin libc-dev-bin libc6 libc6-dev libc6-i386 (Skype)

I’m using Ubuntu 10.10. 64-bit version.

Any help would be appreciated.

(i’m trying to do this from CSIR, Stellenbosch)
In Synaptic Package Manager, under settings, download from (software sources) I have http://ftp/leg.uct.ac.za/ubuntu
When I click Select best server it says No suitable server found, check your internet connection. But I am able to browse websites.

Thanks
 Nick

change repository also fails

When I try to change the repository to http://ftp.sun.ac.za/ftp/ubuntu I also get an error:

W: Failed to fetch http://ftp.sun.ac.za/ftp/ubuntu/dists/maverick/Release.gpg Could not connect to ftp.sun.ac.za:80 (146.232.65.10). - connect (111: Connection refused)

W: Failed to fetch http://ftp.sun.ac.za/ftp/ubuntu/dists/maverick/main/i18n/Translation-en.bz2 Unable to connect to ftp.sun.ac.za:http:

W: Failed to fetch http://ftp.sun.ac.za/ftp/ubuntu/dists/maverick/main/i18n/Translation-en_… Unable to connect to ftp.sun.ac.za:http:

…etc…

Thanks
 Nick

Seem to have some problems sorted out,

It looks like all I need to do was apply my network proxy settings system-wide which I had not done when I entered the manual proxy settings.

I also have the same problem

I also have the same problem & when I apply the manual proxy settings I still get the same error

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