What you may also need to do is go to Settings>Privacy & Security and there should be an option for PixMap after getting the "unidentified developer"-message that you may click "Open Anyway" on. After doing that, it should run normally! Either that or right-click PixMap.plugin and click "Open" just once and it should stop trying to block it from running within After Effects.
Also, PixMap is completely free! You may put $0.00 into the amount to donate and get the download.