Here's the way I did it and it was fairly simple.
First, read what Mongo says @ http://www.mongodb.org/display/DOCS/PHP+Language+Center
When you run sudo pecl install mongo
it's going to install it for the standard OSX PHP installation to use, so if for some reason you're using the built-in Apache installation.
Like you, I like using MAMP. I didn't want to go through figuring how to download Mongo into MAMP, so I just creating a symbolic link in the MAMP PHP Extensions folder and put the extension line in the MAMP php.ini. Restarted MAMP Apache and everything worked beautifully!
A couple things to note, after installing Mongo, note the extensions folder it installed it into then check MAMP php.ini to see where it has its extensions set to be. You'll notice the ending is very similar.