First you define two local variables as reference to the whole sketch and the circumcenter:

| sketch center |
sketch := DrGeoCanvas new fullscreen.

Then you define a triangle by specifying the coordinates of its summits (vertices):

sketch polygon: {-2@0  .  2@0 . 1@3}.

As a note for the reader, -2@0 should be understood as the coordinates (-2;0). It is one way to define 2D coordinates in Smalltalk language. Here we construct the triangle as a polygon with three summits, defined as a list of coordinates separated by the period (.) characters {obj1 . obj2 . obj3}. Later on in the description of the sketch, we refer to the triangle summits by its coordinates, Dr. Geo takes care to find it back in the sketch.

The circumcenter is constructed as the intersection of two perpendicular bisectors:

center := sketch 
        intersectionOf: (sketch perpendicularBisector: -2@0 to: 2@0)
        and: (sketch perpendicularBisector: 2@0 to: 1@3).

Here, it is decided to keep a reference of the circumcenter in the center variable for later use in the construction of the circumscribed circle:

sketch circleCenter: center to: 2@0

The whole scketch description looks like this:

| sketch center |
sketch := DrGeoCanvas new fullscreen.
sketch polygon: {-2@0  .  2@0 . 1@3}.
center := sketch 
        intersectionOf: (sketch perpendicularBisector: -2@0 to: 2@0)
        and: (sketch perpendicularBisector: 2@0 to: 1@3).
sketch circleCenter: center to: 2@0

Copying and pasting this script in a Dr. Geo's workspace, then selecting it and executing with Ctrl-d, will give you the following sketch: circumscribedCircle.png

The curious reader will try to describe the inscribed circle to the triangle! The user manual will be very helpful.