[plt-scheme] DrScheme: Reference to undefined identifier: require

Hello Everybody:
    I am new to Scheme, so please be patient with me.  I have 2 files:
ch2.scm and schemeTests.scm.  I would like to include schemeTests.scm
in the ch2.scm file.

According to the documentation, the include procedure "inlines the
syntax in the file designated by path-spec in place of the include
expression."  http://docs.plt-scheme.org/reference/include.html  This
behavior is what I need.  However, I am having some trouble.
Hopefully, this short interactive session can tell you something.

Welcome to DrScheme, version 4.2.1 [3m].
Language: R5RS; memory limit: 128 megabytes.
> (require scheme/include)
. . reference to undefined identifier: require
> (include "schemeTests.scm")
. . reference to undefined identifier: include
> #lang scheme
. . reference to undefined identifier: module

If I place a call to (require scheme/include) in my source file, I
receive the same error.  However, if I change the language from R5RS
to advanced student custom, I get no complaints about 'require', but
it breaks some of my previously existing code.  What am I doing
wrong?  Your help and consideration is appreciated.

Sincerely,
Daniel