Multigraph is available for download from http://www.sourceforge.net/projects/multigraph.
After you obtain a copy of the multigraph distribution, and unpack it if you downloaded a tar distribution, it will create a directory called "multigraph". Inside that directory there should be a file called "multigraph.jar"; this is a pre-compiled java binary that you can use right away. It includes all the multigraph classes as well as the MultiGraphlet applet. You do not need to compile anything yourself unless you want to.
If you want to compile Multigraph yourself, because you have made changes to the source code, or just because you like to compile things, first make sure you have the "ant" tool installed. Then simply run "ant" when inside the top-level "multigraph" directory. That should build the "multigraph.jar" file from scratch. (You might need to remove, or rename, the existing multigraph.jar file first, to force ant to rebuild it.)