The getting involved page on unity.ubuntu.com has an initial set of instructions to get started.
It looks like you've already branched the code. You can just edit it with whatever editor you prefer (the unity folks don't have a specific IDE, it's just personal preference). After you've made the change you want you can do a
bzr commit and then you need to build Unity to test it:
You don't have to follow the part of the instructions where you pull unity from Launchpad, you've already done that.
After you've fixed what you want you can submit the branch to Launchpad if you want the Unity folks to integrate it.
We have a page full of bitesize bugs for people who are just getting started if they want to tackle something small to get used to the codebase.
If you have specific questions about parts of the unity code just ask them as new questions and tag them with unity and Unity developers will look at them, it will be useful to start building up a bunch of questions and answers about the Unity codebase!