This is something I come across from time to time, and it seems that the procedure is a bit different than it is for Linux, so I thought I’d save the process that I found works (if only so I can find it again later myself.)
Edit ~/.bash_profile (this way you only screw with your own PATH, and no other users’. Say you need to add “/Applications/Graphics/Pixie/bin” to your path, as I did (seriously, getting Pixie to work on OS X is proving to be a massive headache). Let’s also say you need to edit your DYLD_LIBRARY_PATH (Mac’s equivalent of Linux’s LD_LIBRARY_PATH) as you may need to for Pixie. Add the following lines to the end of the file:
Save it, and enter
source ~/.bash_profile in your terminal to make the change effective immediately. This should do the trick in 10.8 Lion. Pixie’s install instructions mention some nonsense about “DYLIB_LIBRARY_PATH” for the Mac – ignore it. Seriously.