Download the latest Openbravo source tarball. Save it anywhere you like, but don't put it in apache's DocumentRoot, nor in tomcat's webapps. What needs to be there will be put there later in the installation process.
Once the download is complete, you can extract the code with the following command:
tar -xjvf openbravo-3.0PR18Q3.tar.bz2
If you are using Windows, make sure you uncompress this file with a tool capable of managing long file names, like 7-zip. Using 7-zip is the recommended option for Windows users.