I pasted the .conf file into Application Support/bitcoin but I need to let the app somehow know that I want to apply the .conf file to it.
No you don't... if your bitcoin.conf file is in that directory, then Bitcoin Core should use it automagically. To confirm that it is, if using GUI, you should be able to goto "Settings -> Options -> Main" and click the "Open Configuration File" button:
It should open the file in your default text editor and you'll see the contents of the file... you should even be able to edit/save it and then when you restart Bitcoin Core the changes will be activated.