Which version of Word?
"Suppress blank lines" is the default anyway in Word 2002 and later. If
necessary you can toggle it on and off in VBA using
ActiveDocument.MailMerge.SuppressBlankLines = True
and
ActiveDocument.MailMerge.SuppressBlankLines = False
or you can re-enable the old Mail Merge Helper using
Tools->Customize->COmmands, select category All COmmands, locate
MailMergeHelper and drag it to a toolbar or menu. Then open the Merge...
option and make the setting.
However, the feature only suppresses blank lines that are formed from at
least one blank MERGEFIELD field, and blank lines are not suppressed if the
line is "inside" another field such as IF. If you are using an ADDRESSBLOCK
field you may also need to scrap that and use individual fields instead.