1. Make sure you are in the folder containing namecoind. Use cd to change directories. You can use TAB when typing to autocomplete directories. For example:
cd /Users/<YOUR USER NAME>/Documents/namecoin
2. Make sure the namecoind binary is actually named namecoind. Mine is named namecoind_osx64 for some reason, but I can't remember if I renamed it.
3. Make sure namecoind is executable. Use chmod to add the execute permission.
4. Start namecoind. Even if you are in the correct directory, simply running namecoind will result in "command not found". You must prefix executables in the current directory with "./" (a single dot refers to the current directory).
Adding the "&" to the command will run it in the background, so you can continue using the terminal. Running namecoind for the first time may prompt you to create a configuration file with an rpc user and password.
5. Then try to issue commands. Here are some example commands, use help to list them all.
./namecoind help
./namecoind getinfo
./namecoind getnewaddress
./namecoind getbalance
6. To shut down namecoind cleanly, use the stop command