What I have done is made a form where the user selects the record he wants to MailMerge. They go to the appropriate number and the Word Document uses a query for the Mail Merge. That query uses the form in its criteria.
For instance, lets say your unique identifier is called "ID" and your form is named "FindRecord." The FindRecord form would have a field for "ID" and then in the query you would have the criteria for the ID field set to: Forms!FindRecord!ID.
I then place a button on the FindRecord form which opens the appropriate word document (The only way I know to do this is use: FollowHyperlink "C:/AppropriateLocation.doc").
When the Word document opens, it will only pull up the records in the query (which will only be the record displayed on the form).