To install third-party filters:
Once you find something you like, simply download and install. If the instructions about how to install those units or compositions uses the standard folder in as a place for Core Image units and Quartz Composer compositions, those will work in Pixelmator. If you happen to have some compositions or units that you would like to play with, put them in to that folder too. You can create your own Quartz Composer or Core Image units filters.