You really don't need a macro for this. Not only that but using a macro can result in unwanted 'Page left blank' pages when you open or print the document using a different active printer driver.

All you need is a field, coded along the lines of:
{IF{=MOD({PAGE},2)}= 1 "{QUOTE 12}¶
This page intentionally left blank¶
"}
to automatically insert a blank page if the page on which they occur is an odd-numbered one.

To get the same effect for an even-numbered page, change the '1' to '0'.

Such a field would be used with ordinary 'Next Page' Section breaks to separate chapters and placed after the final '.' in each chapter (i.e. immediately before the 'Next Page' Section break).

Note: The field brace pairs (i.e. '{ }') for the above example are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac); you can't simply type them or copy & paste them from this message. Nor is it practical to add them via any of the standard Word dialogues. The spaces represented in the field construction are all required. Instead of the ¶ symbols shown in the examples, you should use real paragraph breaks. Simply format the 'This page intentionally left blank' with however much 'before' spacing you require and apply a centred alignment.

As a bonus, the field code solution obviates the need for the .docm format and any macro security/activation/running issues.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]