I run MongoDB on Mac:
Shave:mongodb_simple Logan$ ./bin/mongod -f conf/mongod.conf about to fork child process, waiting until server is ready for connections
This worked for me:
run in terminal
sudo rm -rf mongod.lock export LC_ALL=C
then
sudo mongod --fork --config /xxxx/xx/mongod.conf --logpath /xxx/log/mongodb/mongodb.log