ADVICE: DO NOT EDIT TORRC UNLESS YOU KNOW WHAT YOU'RE DOING!

This is especially true when trying to "block malicious nodes" (e.g. by using Exlude(Exit|Entry)?Nodes) and the like. Why? Let me quote The Tor Project's FAQ:

We recommend you do not use [options altering the node selection algorithm] — they are intended for testing and may disappear in future versions. You get the best security that Tor can provide when you leave the route selection to Tor; overriding the entry / exit nodes can mess up your anonymity in ways we don't understand.

In fact, most valid reasons for wanting to change Tails' stock torrc are covered by the settings available in Vidalia. The only valid options not covered by Vidalia from the top of my head are ReachableAddresses (if your network blocks certain IP address blocks) and LongLivedPorts (in case we've missed some application's long lived connection). If you really need something like that, then (and only then) you should do something like what Tom describes. However, he omits how to make your torrc changes apply to the current Tor instance.

If you don't understand every step in Tom's post I recommend to instead doing the following in a terminal (e.g. Applications -> Accessories -> Terminal):

1. Edit the torrc file as the super user:

gksudo gedit /etc/tor/torrc

2. Apply the new changes to the current tor instance:

sudo service tor restart

(Technically you could to a reload instead of a restart, but restarting is generally more robust against network failure when Tor is bootstrapping which might be why you want to add ReachableAddresses)

Take care now, and I mean it.