Download Pixelmon here.
Forge is needed!
The Forge version is very important to get right. The correct version for Pixelmon 2.3 version is explained below:
MAKE SURE MINECRAFT IS CLOSED BEFORE DOING THE FOLLOWING ACTIONS.
Download the Jar, run the Forge installer and close.
The easiest of all steps is installing Pixelmon. Simply extract the downloaded zip into the .minecraft folder. Afterwards, there should be a database folder and a mods folder with pixelmon.zip contained inside. When you start up Minecraft, check whether, in the mods menu on the main screen, it lists Pixelmon as one of the mods. If it does, Pixelmon is installed! Also make sure that you delete your pixelmon.cfg from your .minecraft/config folder and on your servers if you have already had a previous version of Pixelmon.
This is the exact same but easier! Download the latest Pixelmon mod (will be a .zip file) and just drag and drop the .zip into your mods folder located at /user/Library/Application Support/minecraft/mods. If there is not a mods folder there then simply create one and put the .zip of Pixelmon in there.
When the Forge installation is done, open the new Minecraft launcher and click "Edit Profle", and a pop-up window should appear with lots of settings. Then go down to "Use Version" and click on the drop down menu. There should be an option called "release 1.6.2-Forge220.127.116.112" or something similar. Choose that and click "Save Profile". Then play the game!