1. Download the Minecraft.app (.dmg download )from the Minecraft website here.
2. Open the .dmg and drag Minecraft.app into you Applications folder.
3. Open TextEdit and paste in this
java -d64 -Xms1G -Xmx4096M -jar //Applications/Minecraft.app/Contents/Resources/Java/Bootstrap.jar
Replace 4096M with the amount of RAM you want the launcher to use. Most Macs have at least 8GB memory therefore 4096 ~ 4GB is sufficient. General rule of thumb is to not allocate more than half of your available memory.
4. Once edited save the file as minecraft.command in your applications folder or on your desktop and double click it to launch Minecraft.
5. In the launcher press Edit Profile and select the version of Minecraft Pixelmon is currently on. (1.8.9 at this point). You need to launch that version of the game at least once before proceeding onto the next step.
6. Once loaded close the game and download the forge installer corresponding to the Pixelmon version you will be using from here.
7. Double click the downloaded file (depending on your security preferences you might need to Ctrl+RightClick and press Open). Select "Install Client" and wait for the files to download.
8. Go to your Application Support folder (Cmd+Shift+G and paste in ~/Library/Application Support) and find "minecraft". Create a "mods" folder inside of it and paste the Pixelmon mod into the new mods folder.
9.Launch Minecraft (always using the minecraft.command), go to Edit Profile and from the "Use version" profile select the one that has the word "forge" in it.
At the bottom of the same window, tick JVM Arguments and paste this in
-Xmx4G -Xmn256M -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy
changing the 4G to the same amount you allocated in step 3.
10. Press Save Profile and you're ready to play!
If you wish to add other sidemods or minimaps just paste them into the same mods folder from step 8. Hope that helps!