DBENCH is hosted as a git repository at samba.org so you must make sure you have the git tool installed.
Use git to create a local branch of the official git repository for git and download the tree.
git clone git://git.samba.org/sahlberg/dbench.git dbench
This creates a local subdirectory "dbench" which is a branch of the master repository.
Later when you want to update your branch and resynchronize with the official branch you can use a "git pull" to download all the changes since you branched and resynchronize your local branch with the latest versions of the sources.
Now you need to compile and install DBENCH.
cd dbench ./autogen.sh ./configure make make install
And that should be it. You should now have dbench installed on your system.