Distribute your template by putting it in the Workgroup templates folder.
The users need to have their macro security set to trust installed templates
and Add-Ins. Then your macros will not be challenged. For more on the
different kinds of templates, tabs on the file new dialog, and locations of
templates folders see http://addbalance.com/usersguide/templates.htm.

A lower-tech option is to put a big textbox in the middle of your letter
(formatted to be behind text) that says "This will not work unless macros
are enabled." in big red letters. Have your userform remove the textbox.