@ckeen Well, ctrl-q (or CMD-q) gives poor man's completion, but look at the 'Help'->'Extending the System' help page:

OCompletion
"Provides source code completion as you type"
(Installer ss project: 'OCompletion') install: 'Ocompletion'.
(Smalltalk at: #ECToolSet) register.
(Smalltalk at: #ToolSet) default: (Smalltalk at: #ECToolSet).

BUT: I'd suggest to try this one, just made for you:
https://gist.github.com/krono/c5018c5c02a3f774fbf19126a2793593