Package Manager Help

It's basically saying it needs that lxml library.
You have a ton of choices.
Since you seem to be novice on these things, I'll assume you won't be up to the task (not stupid, just maybe you will find it cumbersome) of compiling from sources.

So you can either try Fink or MacPorts.
I personally love fink but the server(s) have been down as of lately so you'll either have to play a lot or just give up and try MacPorts first.

What is fink or macports? Long story short they'll help you obtain (mostly or all) open source binaries for popular software found in an unix-based OS (Linux, FreeBDS, etc).
They act as a package manager (something that can track down installed things and update/remove/install). There is no such thing as a package manager on OS X, or at least, not from the end-user point of view.

When they suggest you use your package manager and enter some command on the console, they are saying "Use your package manager to install this thing".
Once you install fink/macports you will be able to use a similar command from the command line of OS X to install lxml (and update in the future).
They are really clean and don't leave any mess behind, that is, whatever you install via their commands you can easily uninstall (sometimes it just takes an edit to a file and a send-to-trash of a folder).

Apple ships OS X with some of the software I referred to before (like Python, which Inkscape uses). Unfortunately, their builds (app version if you wanna see it that way) are way behind, so by installing Fink or MacPorts you'll be able to install the latest versions and much more.
There are also other package managers like HomeBrew, but I didn't like them at first look.

Anyway, sorry if there are things I said you already find to be obvious, but hopefully you'll be on your way to have some commandline fun

PD: Here's lxml on MacPorts. You'd need to install (once you learn how) the one that corresponds to your python version (command: python -V).