Postgres installation on MacOSX - problem with psql
Postgres.app has a short documentation here:
http://postgresapp.com/documentation/
which tells among other things:

Configure your $PATH
Postgres.app includes many command line tools. If you want to use
them, you must configure the $PATH variable.
If you are using bash (default shell on OS X), add the following line
to ~/.bash_profile:
export PATH=$PATH:/Applications/Postgres.app/Contents/Versions/9.4/bin
...

Not doing that results in the error message you're getting.