Okay. I need to fix this at some point.
In the meantime, please download and unzip this file (https://dl.dropboxusercontent.com/u/18255196/dartelmni_6_hires.nii.zip) and put it into your lead/templates/dartel/ folder. Re-run again. I’d also advise that you run each step separately to be able to debug a bit better if you’re still unfamiliar with the toolbox.