Here are bzipped tarballs of metalock releases.
A theme is needed to compile metalock. Blue Marble is the default theme in the current release.
Blue Marble - Default
To install metalock, either install the port, or build manually.
Issue the following commands as root:
# cd /usr/ports/x11/metalock
# make deinstall reinstall
Install from source
The (VERY) short answer is:
# cd src/
# make clean install
The long (and better) answer is:
See Installation section in README.