You need to either provide the full path to the 'bmk' executable, or add it's path to your PATH environment variable. The first one is easy. Just type the full path that you installed BlitzMax to before you type 'bmk'. For example, I installed BlitzMax in '/Developer/BlitzMax'. Here's what I would type:
/Developer/BlitzMax/bin/bmk
If you don't want to have to type this each time you want to use 'bmk' than you can also it it to your PATH. The PATH environment variable tells Terminal (technically, 'bash') where to look for executables. Since I installed BlitzMax in '/Developer/BlitzMax', here's how I would add the path to 'bmk' to my PATH environment variable:
echo "export PATH=\$PATH:/Developer/BlitzMax/bin" >> ~/.bash_profile
This adds the quoted line above into the '.bash_profile' file in your home directory, creating one if it does not exist. Whenever 'bash' (the shell which interprets all terminal commands in Mac OS X) starts up, it reads this file, executing each line in the file. This appends the '/Developer/BlitzMax/bin' directory to the PATH environment variable.
After typing this line, you need to close your Terminal window, and open a new one for it to take effect. After that, you should be able to type 'bmk' without providing a full path to the executable. Also, since you're using OS X 10.2, you may need to switch your shell to 'bash'. To do this open Terminal, choose Preferences from the menu, select 'Execute This Command:', and enter '/bin/bash' into the text field provided.
Hope that helps!