If you want to use Postgres.app, you'll probably want to turn off any existing PostgreSQL servers you have run to free up port 5432. Typically these are configured to run via launchd, so you'll need to use launchctl to disable them, e.g.:
sudo launchctl remove /Library/LaunchDaemons/com.edb.launchd.postgresql-9.0.plist
As you can see you need to pass the path to a .plist file. There's a couple of places these can exist, so you can use grep to find them:
grep -ri postgres /Library/LaunchDaemons /System/Library/LaunchDaemons ~/Library/LaunchAgents
