[plt-scheme] trying to learn scheme and use sqlite.ss

Morning,

In DrScheme I enter in the lower window:

> (require (planet "main.ss" ("schematics" "schemeunit.plt" 3 3)))
> (define dbh (open (string->path "mikee.sql3")))
. open: illegal use of signature form in: (open (string->path "mikee.sql3"))

So the require is working (good), but the open is not working. I
don't understand what violation I have in the syntax. To
my so far limited understanding I'm following the code at
<http://coach.cs.uchicago.edu:8080/package-source/jaymccarthy/sqlite.plt/3/7/sqlite-test.ss>.

What am I missing here?

Mike