Keep in mind that you are getting optifine for 1.7.2 meaning that you won't be able to take screenshots like normal.

I'm also pretty sure that you just need to download it and run it and it'll make a new optifine profile for you.

 

1. Go to http://optifine.net/adloadx.php?f=OptiFine_1.7.2_HD_U_A6.jar
2. Open the file
3. Install
4. Open minecraft and look on the left where it says "profile:"
5. Change it to Optifine
6. Click play

This is the instructions for Windows. I think that it should be the same for mac. Sorry if it isn't.