1. Install Monit as root:

 ./configure --bindir=/opt/local/bin/ --mandir=/opt/local/man/ \
   --sysconfdir=/opt/local/etc/ 
 make
 make install

2. Edit startup configuration:

/Library/LaunchDaemons/com.tildeslash.monit.plist

 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" \
    "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
 <dict>
       <key>Label</key>
       <string>com.tildeslash.monit</string>
       <key>ProgramArguments</key>
       <array>
               <string>/opt/local/bin/monit</string>
               <string>-c</string>
               <string>/opt/local/etc/monitrc</string>
       </array>
       <key>RunAtLoad</key>
       <true/>
 </dict>
 </plist>

3. Load:

 launchctl load -w /Library/LaunchDaemons/com.tildeslash.monit.plist