That's really cool that you're following the dev of the github repository. You've probably seen that GitHub broadcasts the source commits to RSS feeders, so you can almost figure out the new features being added as they are checked in.
Omar and I spent a lot of time trying to figure out how to manage remote source code contributions for both the IDE code base and the hardware board cores.
Here's how we did it:
<!-- Get Illuminato X Machina Core -->
<move file="hardware/cores/SFB" tofile="hardware/cores/ixm"/>
The IXM is an example of a remote core, hosted externally, the source code is grabbed from a web URL.
So I've seen your error before when I forgot to get the latest updates from github.
Pull the latest from GitHub:
$ git pull
Then you can build and run the IDE right from your win/mac/linux by issuing the following command:
Build Source and Run the IDE:
$ ant dist.all run.all
Hope that helps.