fixed instructions failing to mention to start the app

docs/Developing-Wallace-(setup-guide).md

@@ -37,12 +37,22 @@ On Ubuntu, follow the instructions under the heading "Installation" [here](https
 
 ## Create the Database
 
-After installing Postgres, you will need to create a database for your experiments to use. Run the following command from the comand line:
+After installing Postgres, you will need to create a database for your experiments to use. First, open the Postgres.app in any way you like (e.g., <kbd>⌘</kbd>+<kbd>␣</kbd> (command and space) and type `Postgres`). Then, run the following command from the comand line:
 
 ```
 psql -c 'create database wallace;' -U postgres
 ```
 
+If you get an error like: 
+
+```
+psql: could not connect to server: No such file or directory
+    Is the server running locally and accepting
+    connections on Unix domain socket "/tmp/.s.PGSQL.5432"?
+```
+
+Then, you probably forgot to start the app.
+
 ## Set up a virtual environment
 
 Set up a virtual environment by running the following commands: