There is a bug in Linux Mint 12 RC related to adding PPAs: when you add a PPA using "add-apt-repository", the .list file (under /etc/apt/sources.list.d/) uses "lisa" as the code name instead of "oneiric". Since the PPAs are not using the Ubuntu code name, they won't work and when running "sudo apt-get update", you'll get a "Failed to fetch [...] 404 Not Found" error.
A work-around is to edit the .list files under /etc/apt/sources.list.d/ and replace "lisa" with "oneiric", but to make it easier, I've updated Y PPA Manager with a work-around for this bug, so adding a PPA through Y PPA Manager should work as usual, without having to tweak anything else.
To install the latest Y PPA Manager in Linux Mint 12 RC, use the commands below:
sudo add-apt-repository ppa:webupd8team/y-ppa-manager sudo sed -i 's/lisa/oneiric/g' /etc/apt/sources.list.d/webupd8team-y-ppa-manager-lisa.list sudo apt-get update sudo apt-get install y-ppa-manager
(Since installing Y PPA Manager requires adding a PPA, the second command above fixes the Y PPA Manager .list file)
Extra tip: if you've already added some PPAs, fix them all by using the command below (make a backup firstly!):
sudo sed -i 's/ lisa / oneiric /g' /etc/apt/sources.list.d/*.list
Update: the bug has been fixed. Run an update and adding PPAs should work as normal.