Can't add/remove programs, VLC install doesn't work

I've had my One for a few weeks now, and I've enabled advanced mode and updated Firefox. I tried to install VLC using a post on this forum, however, and it didn't work for me. I had it installed once before but due to an error had to use the restore disc it came with. I tried to install it the same way I did the first time, and when i download the program from VLC's website, it downloads fine but won't execute. When I click on it the window saying "Installing the following packages" is empty, and I cannot click "apply". I tried installing it directly from the terminal, and a message comes up saying: "warning: /var/tmp/rpm-xfer.SoTPEv: Header V3 DSA signature: NOKEY, key ID a109b1ec". Does anyone know what this means, and how I can get around it to install VLC?

Also, I'm not sure if this is related or not, but I cannot access my Package Manager. I have advanced mode activated, but when I right click and select "Add/Remove Programs" and the Retrieving Software Information status bar comes up, it get's stuck about a quarter of the way down. The hard drive light goes off, and the window freezes - can't exit it or hit "cancel".

Can anyone here help? Thanks so much!