You'll need to use at least version 4.0 of the iPhone SDK
iReddit requires TouchJSON and a special branch of the Three20 project to build.
In the parent directory of the project directory, you can check out the requirements with the following commands:
hg clone https://touchcode.googlecode.com/hg/ touchcode
(go make a cup of tea)
ln -s touchcode/TouchJSON TouchJSON git clone http://github.com/alastairstuart/three20.git
So your directory structure should look like this:
. <parent directory> |-- iReddit |-- three20 |-- touchcode '-- TouchJSON (symlink -> touchcode/TouchJSON)
You should then be able to build iReddit.