In Oracle APEX 5.1 you can install Quick SQL by going to the Packaged Apps section and install it from there:
I really love Quick SQL as it allows me to build my data model very fast, but also shows me the structure of the tables in a very efficient way. That is why I created a script that can reverse engineer existing tables into the Quick SQL format.
From Oracle APEX 5.2 onwards you won't find Quick SQL in the packaged app section anymore... but no worries, it's not gone, it's now built-in the APEX framework itself :)
Go to SQL Workshop - SQL Scripts:
Hit the Quick SQL button:
Here you have Quick SQL :)
You can run your script after you save, straight from this interface.
Note: the screenshots are taken from Oracle APEX 5.2 Early Adopter, so things might change in the final release of APEX 5.2.