I was looking around for this as well. It wasn't clear at first, but I found these pages, which were helpful:

http://nodebox.net/code/index.php/cornu.path()
http://nodebox.net/code/index.php/cornu.drawpath()

One things which I think might be an error in the path() doc is that it says it returns a list of coords between 0.0 and 1.0, describing a point's position relative to the box, but it appears to return absolute positions.

For example:
#########
size(800,800)

cornu = ximport("cornu")
aPath = cornu.path([(.2, .5), (.6, .7), (.9, .1), (.5, .5)], tweaks=50)
for point in aPath:
print '%d : %d' % (point.x, point.y)
##########

the above prints out:
160 : 400
162 : 402
164 : 405
167 : 408
...

The documentation is perhaps a bit unclear. The path() command accepts relative coordinates. The points returned are absolute.

I've changed the documentation to:

"Returns a path object from a given list of (x,y)-tuples. The x and y coordinates in this list range between 0.0 and 1.0, describing a point's position relative to the drawing area's size(). The points in the returned path object are absolute positions."

Hopefully that is more clear.

As for a tutorial, the cornu library is not that hard. Try Mark's above example:

size(800,800)
 
cornu = ximport("cornu")
aPath = cornu.path([(.2, .5), (.6, .7), (.9, .1), (.5, .5)], tweaks=50)
 
nofill()
stroke(0)
drawpath(aPath)