You can follow the instructions linked above to install the tar.gz distribution of VoltDB Enterprise Edition, which includes a 30-day trial license. You can download the precompiled binaries for Enterprise Edition here: http://voltdb.com/download/enterprise-download/
The Enterprise Edition is available as a .tar.gz which you simply un-tar to install, or a .rpm and .deb packages which you can install on RHEL, CentOS or Ubuntu using their respective package management tools.
The Community Edition is available as source code on Github. Pre-compiled binaries of Community Edition are not currently available for download on voltdb.com. To build community edition, you will need to follow the instructions in the wiki here: https://github.com/VoltDB/voltdb/wiki/Building-VoltDB
If you want to build a released version of Community Edition, rather than cloning the github project (master branch), you can download and unzip the released version of the source code from here: https://github.com/VoltDB/voltdb/releases
, or you can clone one of the release branches listed here: https://github.com/VoltDB/voltdb/branches