This was a frustrating process for me, but I finally got it.
Watch this video. It’s good and may give you everything you need.
If not (there were a few extra things I needed to do) read on.
1. Download minecraft forge.
Goto http://files.minecraftforge.net/ and download the ‘Installer’ for the most recent recommended version (1.6.4 at this time of writing).
To skip the adds, click the ‘*’ beside the word ‘Installer’ on the ‘1.6.4-Recommended’ and download the jar file (it will save to your downloads directory).
2. Save old profile.
The next thing we are going to do is save your current minecraft configuration, before blowing it all away with the new minecraft forge client.
Open up your ‘finder’ and click ‘Go to Folder…’
Then type in exactly exactly what you see below into the textfield and hit the ‘Go’ button.
This will take your to your minecraft application directory where your profile is stored (sorry, I forget to mention you will need to install minecraft on your Mac first before installing forge).
Once there drag and drop these files and folders to your desktop.
Then blow everything else away in the directory.
3. Create forge profile.
Now start your minecraft application. This is going to download the latest minecraft version.
Now here is where things differ from the video. If your minecraft client version (1.7.3), is different from the latest forge version (1.6.4), it’s not going to work (at least not yet).
We need to create a 1.6.4 profile (the same version as forge) and then things will work.
So if your latest client version differs from the forge version, create a new profile matching them up.
For example to to create a 1.6.4 profile, click ‘Profile Editor’ at the top of your login screen.
Then click ‘New Profile’.
Then file in the profile name (pick whatever you want, maybe put 1.6.4 in there so you know the version), and then select ‘release 1.6.4’ for the Use Version, and then ‘Save Profile’.
Now you have a compatible client that is going to work with your forge!
No you can go back to your login screen, select your new 1.6.4 profile, and login.
Click ‘Quit Game’ and shut down your minecraft launcher (Command-Q) or right click quit in your application toolbar.
4. Install forge client
Drop the files we dragged to the desktop earlier, back into the minecraft application folder we deleted everything from earlier.
Select ‘Apply to All’ and then ‘Replace’
Then double click your minecraft forge jar file that we downloaded at the start (downloads directory).
If you get this warning message
Open your ‘Finder’ and go to your ‘Downloads’ directory manually and ‘right-click’ on the jar file and select open
The click ‘Open’ to run the installer
Click install client, and make sure that the path in the install directory points to the directory where we stored all your profile stuff. It should look something like this:
We are almost there! Click ‘OK’.
You should then see a completion window that says ‘Successfully installed client profile …’
5. Create client forge profile.
Open up minecraft. In the profile areas, a profile option should now appear!
Select that. Click the ‘Play’ button. And congrats! You should be in.
You can tell your are running forge by the data in the bottom left hand corner.
6. Installing mods.
Forge mods go in the ‘mods’ folder in that directory we where looking at earlier where we did all our profile stuff.
This is where you put your forge mods.
That’s it for now! Happy forging!