PostgresApp
Installing plugins
Sure, you can!
The general recipe is to get the extension's source, it's dependencies, make sure you have set the path correctly, so that PostgresApp's pg_config is reachable and run the usual ./configure && make && make install of the extension. If that was successful, you should be able to connect to your database and run a CREATE EXTENSION whatever;. The specific steps may differ from one to an other extension, but generally that's it. You can also install the pgxn client and use that to assist in the process.
If you would give some more details on what specific extension you want to install and where in the process you have trouble, the chance would be higher to find some user that as has experiences with it that can be shared. Without that, I can only recommend to follow the instructions of the extension authors.
