If you install the sensu-server from source and starting it from the bin folder, make sure to pass in the -e flag specifying the directory for sensu extensions
(DEV) sensuserver sensu # bin/sensu-server -h
Usage: sensu-server [options]
-h, --help Display this message
-V, --version Display version
-c, --config FILE Sensu JSON config FILE
-d, --config_dir DIR[,DIR] DIR or comma-delimited DIR list for Sensu JSON config files
-e, --extension_dir DIR DIR for Sensu extensions
-l, --log FILE Log to a given FILE. Default: STDOUT
-L, --log_level LEVEL Log severity LEVEL
-v, --verbose Enable verbose logging
-b, --background Fork into the background
-p, --pid_file FILE Write the PID to a given FILE
(DEV) sensuserver sensu # bin/sensu-server -d /usr/local/sensu/etc/sensu -e /usr/local/sensu/etc/sensu/extensions -l sensuserver.log -b