Once you clone the repo just run an ant build and then add the folder to your path in order to have atdown accessible everywhere. Please submit bugs to the github bug tracker:
git clone https://github.com/AcademicTorrents/AcademicTorrents-Downloader
cd AcademicTorrents-Downloader
ant
To add the tool to your path type:
echo -e "\nPATH=\$PATH:$(pwd)" >> ~/.bashrc
source ~/.bashrc