On a unix-like os, saying './arduino-serial' instructs the shell to run a program found in the current directory, rather than to search the path for that command. So this would only work if you are currently in the directory where that is found.
You can try it without the ./ - just type 'arduino-serial' and the rest of the command line and see if it is found then.
If not, you would have to figure out where this program comes from and where it lives that's not in the path, or how to install it.
However, there's no real requirement that you use that program. If you are using the arduino IDE, there's probably a serial monitor available in the drop down menu.
And surely there are many generic terminal emulator programs available for the Macintosh. You just need one where you can put in the special device string for the serial port, and have it not send modem control AT commands (since the arduino is not a modem). If it twiddles with flow control lines, you may have to wait a bit after starting it for the bootloader to time out and your sketch to start up.
will 'connect' you to the Arduino, assuming the arduino is '/dev/tty.usbmodem621'. Then whatever you type in terminal is sent directly to the Arduino. To exit out of screen, you have to type Ctrl-A then Ctrl-\.
If using those data-only solutions you may need to use something like stty to set the baud rate for the computer end if it hasn't been previously set by running something such as the arduino IDE since boot.
There may also be tty control bit settings which could cause unintended auto resets to the bootloader on port open/close.