No extra field braces are required. You need only insert the tab stops in the positions where you want the alignment to occur, just as you would in an ordinary document. Don't be concerned with whether they mergefields align correctly, as they're quite possibly going to be wider than the actual data.
For your purposes, you could use the two {SET Tot1 0} fields, the {SET Tot1 {=Tot1+{MERGEFIELD Sales}}} field (all in red), plus the {=Tot1 \# $,0} field (with or without the table, depending on what you're doing table-wise). Note that, because you now have two fields for the true condition in the initial IF test, they need to be wrapped in double-quotes, thus:
{IF{MERGESEQ}= 1 "{SET Key ""}{SET Tot1 0}" {SET LastKey {REF Key}}}
In the attachment, which is based directly on the tutorial, I've implemented the totals for a merge using a single key - using 'Total' instead of 'Tot1'. If you simply connect it to your copy of the tutorial's datasource file, you should be able to generate the merge.

Attached Files

Mailmerge Main Document.doc (33.5 KB, 35 views)

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