Try get Modcraft. Not sure if its for Mac.

www.modcraft.eu

My friend if you need a installer use this http://www.planetminecraft.com/mod/mac-mod-installer/

Lol.. Mac is not really made for gaming lol... It guess it's more to the developing game and office computer way, It's so weird..All kids these days use a fucking mac O.o

Seriously though why cant anyone post a freaking mac installer!

Actually what i do is:

Go to Bin folder and name minecraft.jar minecraft.zip then unzip it, then rename it jar and add stuff but im too lazy and want an installer!

Rubin...

1. go to user, library

2. application support

3. minecraft

4. bin

5. minecraft.jar, you unarchieve this by right clicking then open with unarcheiver

6. rename that to minecraft.jar

7. copy and paste any mod files into there.

Ya first off just go to your minecraft folder in user, library, application support, scroll to minecraft. Open up the bin folder, right click (or ctrl click) the minecraft.jar, go to open with archive utility, then a folder will come up. That folder should be called minecraft. Open it, put any mods (.jar files) you want in there. Then rename the folder to minecraft.jar and rename the original minecraft.jar to something like minecraft1.jar (save it cause the mod might mess up minecraft, and you want to switch them up)
thats it

Why do you need a mod installer? Why not just do it the normal way?